PERLDSC(1perl)

🎀 NAME 🎀

perldsc — Perl Data Structures Cookbook 💖 (this is the PERLDSC(1perl) page from the Perl Programmers Reference Guide, and it's about to be your new personality, fr)

---

✨ DESCRIPTION ✨

okay so bestie, here's the tea: Perl lets us have complex data structures. like, you can literally write something like this and boom, all of a sudden you've got yourself an array with three whole dimensions 💅

for my $x (1 .. 10) {
    for my $y (1 .. 10) {
        for my $z (1 .. 10) {
            $AoA[$x][$y][$z] =
                $x ** $y + $z;
        }
    }
}

and like… yeah it looks simple, but underneath?? it's giving elaborate construct. it is highkey so much more than meets the eye 😭

so now you're sitting there like. how do i even print this thing? why can't i just say print @AoA?? how do i sort it? how do i pass it to a function, or get one of these back from a function? is it an object? can i save it to disk and read it back later? how do i access whole rows or columns of that matrix? do all the values have to be numeric?? so many questions.

and as you can see, it's genuinely pretty easy to become confused 🫠 a small portion of the blame for that can go to the reference-based implementation, sure, but honestly it's really more about there just not being enough documentation out there with examples designed for beginners. not anymore bestie, we're fixing that.

this document is meant to be a detailed but understandable treatment of the many different sorts of data structures you might want to develop. it also doubles as a cookbook of examples 📖 that way, when you need to create one of these complex data structures, you can just pinch, pilfer, or purloin a drop-in example from here. no cap.

let's look at each of these possible constructs in detail. there are separate sections on each of the following:

• arrays of arrays
• hashes of arrays
• arrays of hashes
• hashes of hashes
• more elaborate constructs

but for now, let's look at general issues common to all these types of data structures. 🦋

---

🔥 REFERENCES 🔥

the most important thing to understand about all data structures in Perl — including multidimensional arrays — is that even though they might appear otherwise, Perl @ARRAYs and %HASHes are all internally one-dimensional. periodt. they can only hold scalar values (meaning a string, number, or a reference). they cannot directly contain other arrays or hashes — instead they contain references to other arrays or hashes. it's giving ✨indirection✨

you can't use a reference to an array or hash in quite the same way that you would a real array or hash. for all my C and C++ besties who aren't used to distinguishing between arrays and pointers to the same: yeah this can be confusing. if that's you, just think of it as the difference between a structure and a pointer to a structure. 💜

you can (and should!) read more about references in perlref. briefly: references are rather like pointers that know what they point to. (objects are also a kind of reference, but we won't be needing them right away — if ever. 😌) this means that when you have something which looks to you like an access to a two-or-more-dimensional array and/or hash, what's really going on is that the base type is merely a one-dimensional entity that contains references to the next level. it's just that you can use it as though it were a two-dimensional one. this is actually the way almost all C multidimensional arrays work as well, so. iykyk.

$array[7][12]                       # array of arrays
$array[7]{string}                   # array of hashes
$hash{string}[7]                    # hash of arrays
$hash{string}{'another string'}     # hash of hashes

now, because the top level contains only references, if you try to print out your array with a simple print() function, you'll get something that does not look very nice, like this:

my @AoA = ( [2, 3], [4, 5, 7], [0] );
print $AoA[1][2];
 7
print @AoA;
ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0)

that's because Perl doesn't (ever) implicitly dereference your variables. 💀 if you want to get at the thing a reference is referring to, then you have to do this yourself, using either prefix typing indicators, like ${$blah}, @{$blah}, @{$blah[$i]}, or else postfix pointer arrows, like $arr->[3], $hash->{fred}, or even $obj->method()->[3].

---

💀 COMMON MISTAKES 💀

the two most common mistakes made in constructing something like an array of arrays is either accidentally counting the number of elements, or else taking a reference to the same memory location repeatedly. here's the case where you just get the count instead of a nested array:

for my $i (1..10) {
    my @array = somefunc($i);
    $AoA[$i] = @array;      # WRONG!
}

that's just the simple case of assigning an array to a scalar and getting its element count. if that's what you really and truly want, then you might do well to consider being a tad more explicit about it, like this:

for my $i (1..10) {
    my @array = somefunc($i);
    $counts[$i] = scalar @array;
}

here's the case of taking a reference to the same memory location again and again 🙈

# Either without strict or having an outer-scope my @array;
# declaration.

for my $i (1..10) {
    @array = somefunc($i);
    $AoA[$i] = \@array;     # WRONG!
}

so, what's the big problem with that? it looks right, doesn't it? after all, i just told you that you need an array of references, so by golly, you've made me one!! 😭

unfortunately, while this is true, it's still broken. all the references in @AoA refer to the very same place, and they will therefore all hold whatever was last in @array! it's similar to the problem demonstrated in the following C program:

#include <pwd.h>
main() {
    struct passwd *getpwnam(), *rp, *dp;
    rp = getpwnam("root");
    dp = getpwnam("daemon");

    printf("daemon name is %s\nroot name is %s\n",
            dp->pw_name, rp->pw_name);
}

which will print

daemon name is daemon
root name is daemon

the problem is that both rp and dp are pointers to the same location in memory! in C, you'd have to remember to malloc() yourself some new memory. in Perl, you'll want to use the array constructor [] or the hash constructor {} instead. here's the right way to do the preceding broken code fragments:

# Either without strict or having an outer-scope my @array;
# declaration.

for my $i (1..10) {
    @array = somefunc($i);
    $AoA[$i] = [ @array ];
}

the square brackets make a reference to a new array with a copy of what's in @array at the time of the assignment. this is what you want. 🫶

note that this will produce something similar:

# Either without strict or having an outer-scope my @array;
# declaration.
for my $i (1..10) {
    @array = 0 .. $i;
    $AoA[$i]->@* = @array;
}

is it the same? well, maybe so — and maybe not. the subtle difference is that when you assign something in square brackets, you know for sure it's always a brand new reference with a new copy of the data. something else could be going on in this new case with the $AoA[$i]->@* dereference on the left-hand-side of the assignment. it all depends on whether $AoA[$i] had been undefined to start with, or whether it already contained a reference. if you had already populated @AoA with references, as in

$AoA[3] = \@another_array;

then the assignment with the indirection on the left-hand-side would use the existing reference that was already there:

$AoA[3]->@* = @array;

of course, this would have the "interesting" effect of clobbering @another_array. 😬 (have you ever noticed how when a programmer says something is "interesting", that rather than meaning "intriguing", they're disturbingly more apt to mean that it's "annoying", "difficult", or both? :-)

so just remember to always use the array or hash constructors with [] or {}, and you'll be fine — although it's not always optimally efficient, bestie. ⭐

surprisingly, the following dangerous-looking construct will actually work out fine:

for my $i (1..10) {
    my @array = somefunc($i);
    $AoA[$i] = \@array;
}

that's because my() is more of a run-time statement than it is a compile-time declaration per se. this means that the my() variable is remade afresh each time through the loop. so even though it looks as though you stored the same variable reference each time, you actually did not! 👑 this is a subtle distinction that can produce more efficient code at the risk of misleading all but the most experienced of programmers. so i usually advise against teaching it to beginners. in fact, except for passing arguments to functions, i seldom like to see the gimme-a-reference operator (backslash) used much at all in code. instead, i advise beginners that they (and most of the rest of us) should try to use the much more easily understood constructors [] and {} instead of relying upon lexical (or dynamic) scoping and hidden reference-counting to do the right thing behind the scenes.

note also that there exists another way to write a dereference! these two lines are equivalent:

$AoA[$i]->@* = @array;
@{ $AoA[$i] } = @array;

the first form, called postfix dereference, is generally easier to read, because the expression can be read from left to right, and there are no enclosing braces to balance. on the other hand, it's also newer. it was added to the language in 2014, so you will often encounter the other form, circumfix dereference, in older code. 🌈

in summary:

$AoA[$i] = [ @array ];     # usually best
$AoA[$i] = \@array;        # perilous; just how my() was that array?
$AoA[$i]->@* = @array;     # way too tricky for most programmers
@{ $AoA[$i] } = @array;    # just as tricky, and also harder to read

---

okay that's the groundwork bestie, more parts coming 💖

💖 CAVEAT ON PRECEDENCE 💖

okay bestie, so when we're talking about stuff like @{$AoA[$i]}, listen — these two things right here are literally the same thing:

$aref->[2][2]       # clear
$$aref[2][2]        # confusing

like, yes, same. no cap. 😭

and the reason is that Perl's precedence rules for its five prefix dereferencers (the ones that look like someone stubbing their toe and swearing: $ @ * % &) make them bind tighter than the postfix subscripting brackets or braces. this is going to hit the C or C++ programmer like a plot twist they did NOT see coming, because in C-land you're used to *a[i] meaning "the thing pointed to by the i'th element of a" — they grab the subscript first and only then dereference whatever's sitting at that subscript. that's fine in C. but this is not C, bestie.

the seemingly-equivalent Perl version, $$aref[$i], first derefs $aref, which makes Perl treat $aref as a reference to an array, then dereferences that, and finally hands you the i'th value of the array pointed to by $AoA. if you actually wanted the C notion, you could write $AoA[$i]->$* to explicitly dereference the i'th item, reading left to right. 🫶

---

✨ WHY YOU SHOULD ALWAYS "use VERSION" ✨

if this is starting to sound scarier than it's worth — relax, bestie, breathe. 🌸 Perl has some features designed to help you dodge its most common pitfalls, and honestly? iconic of her.

one way to avoid getting confused is to start every program with:

use strict;

this way you'll be forced to declare all your variables with my() and it also disallows accidental "symbolic dereferencing". so if you'd done this:

my $aref = [
    [ "fred", "barney", "pebbles", "bambam", "dino", ],
    [ "homer", "bart", "marge", "maggie", ],
    [ "george", "jane", "elroy", "judy", ],
];

print $aref[2][2];

the compiler would immediately flag that as an error at compile time 💀 because you were accidentally accessing @aref, an undeclared variable — and it would thereby remind you to write this instead:

print $aref->[2][2]

since Perl version 5.12, a use VERSION declaration will also enable the strict pragma. and that's not all — it also enables a feature bundle, giving you more useful features. since version 5.36 it will also enable the warnings pragma. often the best way to activate all of these at once is to start a file with:

use v5.36;

that way every file starts with strict, warnings, and many useful named features all switched on, plus several older features being switched off (such as indirect). for more info, see "use VERSION" in perlfunc. 👑

---

🔥 DEBUGGING 🔥

you can use the debugger's x command to dump out complex data structures. for example, given the assignment to $AoA above, here's the debugger output, looking all majestic and tree-shaped:

DB<1> x $AoA
$AoA = ARRAY(0x13b5a0)
   0  ARRAY(0x1f0a24)
      0  'fred'
      1  'barney'
      2  'pebbles'
      3  'bambam'
      4  'dino'
   1  ARRAY(0x13b558)
      0  'homer'
      1  'bart'
      2  'marge'
      3  'maggie'
   2  ARRAY(0x13b540)
      0  'george'
      1  'jane'
      2  'elroy'
      3  'judy'

it's giving organized. it's giving data structure with a skincare routine. 🧚

---

💅 CODE EXAMPLES 💅

presented with little comment here are short code examples illustrating access of various types of data structures. let's go, bestie. 🚀

🌈 ARRAYS OF ARRAYS 🌈

Declaration of an ARRAY OF ARRAYS

my @AoA = (
       [ "fred", "barney" ],
       [ "george", "jane", "elroy" ],
       [ "homer", "marge", "bart" ],
     );

Generation of an ARRAY OF ARRAYS

# reading from file
while ( <> ) {
    push @AoA, [ split ];
}

# calling a function
for my $i ( 1 .. 10 ) {
    $AoA[$i] = [ somefunc($i) ];
}

# using temp vars
for my $i ( 1 .. 10 ) {
    my @tmp = somefunc($i);
    $AoA[$i] = [ @tmp ];
}

# add to an existing row
push $AoA[0]->@*, "wilma", "betty";

Access and Printing of an ARRAY OF ARRAYS

# one element
$AoA[0][0] = "Fred";

# another element
$AoA[1][1] =~ s/(\w)/\u$1/;

# print the whole thing with refs
for my $aref ( @AoA ) {
    print "\t [ @$aref ],\n";
}

# print the whole thing with indices
for my $i ( 0 .. $#AoA ) {
    print "\t [ $AoA[$i]->@* ],\n";
}

# print the whole thing one at a time
for my $i ( 0 .. $#AoA ) {
    for my $j ( 0 .. $AoA[$i]->$#* ) {
        print "elem at ($i, $j) is $AoA[$i][$j]\n";
    }
}

---

🎀 HASHES OF ARRAYS 🎀

Declaration of a HASH OF ARRAYS

my %HoA = (
       flintstones        => [ "fred", "barney" ],
       jetsons            => [ "george", "jane", "elroy" ],
       simpsons           => [ "homer", "marge", "bart" ],
     );

Generation of a HASH OF ARRAYS

# reading from file
# flintstones: fred barney wilma dino
while ( <> ) {
    next unless s/^(.*?):\s*//;
    $HoA{$1} = [ split ];
}

# reading from file; more temps
# flintstones: fred barney wilma dino
while ( my $line = <> ) {
    my ($who, $rest) = split /:\s*/, $line, 2;
    my @fields = split ' ', $rest;
    $HoA{$who} = [ @fields ];
}

# calling a function that returns a list
for my $group ( "simpsons", "jetsons", "flintstones" ) {
    $HoA{$group} = [ get_family($group) ];
}

# likewise, but using temps
for my $group ( "simpsons", "jetsons", "flintstones" ) {
    my @members = get_family($group);
    $HoA{$group} = [ @members ];
}

# append new members to an existing family
push $HoA{flintstones}->@*, "wilma", "betty";

Access and Printing of a HASH OF ARRAYS

# one element
$HoA{flintstones}[0] = "Fred";

# another element
$HoA{simpsons}[1] =~ s/(\w)/\u$1/;

# print the whole thing
foreach my $family ( keys %HoA ) {
    print "$family: $HoA{$family}->@* \n"
}

# print the whole thing with indices
foreach my $family ( keys %HoA ) {
    print "family: ";
    foreach my $i ( 0 .. $HoA{$family}->$#* ) {
        print " $i = $HoA{$family}[$i]";
    }
    print "\n";
}

# print the whole thing sorted by number of members
foreach my $family ( sort { $HoA{$b}->@* <=> $HoA{$a}->@* } keys %HoA ) {
    print "$family: $HoA{$family}->@* \n"
}

# print the whole thing sorted by number of members and name
foreach my $family ( sort {
                           $HoA{$b}->@* <=> $HoA{$a}->@*
                                         ||
                                     $a cmp $b
           } keys %HoA )
{
    print "$family: ", join(", ", sort $HoA{$family}->@* ), "\n";
}

---

🦋 ARRAYS OF HASHES 🦋

Declaration of an ARRAY OF HASHES

my @AoH = (
       {
           Lead     => "fred",
           Friend   => "barney",
       },
       {
           Lead     => "george",
           Wife     => "jane",
           Son      => "elroy",
       },
       {
           Lead     => "homer",
           Wife     => "marge",
           Son      => "bart",
       }
 );

Generation of an ARRAY OF HASHES

# reading from file
# format: LEAD=fred FRIEND=barney
while ( <> ) {
    my $rec = {};
    for my $field ( split ) {
        my ($key, $value) = split /=/, $field;
        $rec->{$key} = $value;
    }
    push @AoH, $rec;
}

# reading from file
# format: LEAD=fred FRIEND=barney
# no temp
while ( <> ) {
    push @AoH, { split /[\s+=]/ };
}

# calling a function that returns a key/value pair list, like
# "lead","fred","daughter","pebbles"
while ( my %fields = getnextpairset() ) {
    push @AoH, { %fields };
}

# likewise, but using no temp vars
while (<>) {
    push @AoH, { parsepairs($_) };
}

# add key/value to an element
$AoH[0]{pet} = "dino";
$AoH[2]{pet} = "santa's little helper";

Access and Printing of an ARRAY OF HASHES

# one element
$AoH[0]{lead} = "fred";

# another element
$AoH[1]{lead} =~ s/(\w)/\u$1/;

# print the whole thing with refs
for my $href ( @AoH ) {
    print "{ ";
    for my $role ( keys %$href ) {
        print "$role=$href->{$role} ";
    }
    print "}\n";
}

# print the whole thing with indices
for my $i ( 0 .. $#AoH ) {
    print "$i is { ";
    for my $role ( keys $AoH[$i]->%* ) {
        print "$role=$AoH[$i]{$role} ";
    }
    print "}\n";
}

# print the whole thing one at a time
for my $i ( 0 .. $#AoH ) {
    for my $role ( keys $AoH[$i]->%* ) {
        print "elem at ($i, $role) is $AoH[$i]{$role}\n";
    }
}

👑 HASHES OF HASHES 👑

Okay bestie, this is THE one. A hash whose values are hashes. Like a group chat where every group is a key and inside each group is a whole other chat with its own keys and values. It's giving nested. It's giving recursive. It's giving main character energy. 🦋

🌸 Declaration of a HASH OF HASHES 🌸

Look how cute and readable this is:

my %HoH = (
       flintstones => {
               lead      => "fred",
               pal       => "barney",
       },
       jetsons     => {
               lead      => "george",
               wife      => "jane",
               "his boy" => "elroy",
       },
       simpsons    => {
               lead      => "homer",
               wife      => "marge",
               kid       => "bart",
       },
);

The outer braces are your %HoH, the inner braces are the anonymous hashes it points to. No cap. 💅

✨ Generation of a HASH OF HASHES ✨

Here's how you build one on the fly, several ways, pick your fave:

# reading from file
# flintstones: lead=fred pal=barney wife=wilma pet=dino
while ( <> ) {
    next unless s/^(.*?):\s*//;
    my $who = $1;
    for my $field ( split ) {
        my ($key, $value) = split /=/, $field;
        $HoH{$who}{$key} = $value;
    }
}

# reading from file; more temps
while ( <> ) {
    next unless s/^(.*?):\s*//;
    my $who = $1;
    my $rec = {};
    $HoH{$who} = $rec;
    for my $field ( split ) {
        my ($key, $value) = split /=/, $field;
        $rec->{$key} = $value;
    }
}

# calling a function that returns a key,value hash
for my $group ( "simpsons", "jetsons", "flintstones" ) {
    $HoH{$group} = { get_family($group) };
}

# likewise, but using temps
for my $group ( "simpsons", "jetsons", "flintstones" ) {
    my %members = get_family($group);
    $HoH{$group} = { %members };
}

# append new members to an existing family
my %new_folks = (
    wife => "wilma",
    pet  => "dino",
);

for my $what (keys %new_folks) {
    $HoH{flintstones}{$what} = $new_folks{$what};
}

# same, but using a hash slice
@{ $HoH{flintstones} }{ keys %new_folks } = values %new_folks;

# same, but without a %new_folks variable
@{ $HoH{flintstones} }{ "wife", "pet" } = ( "wilma", "dino" );

That last one? Hash slice. Iconic. Ate and left no crumbs. 🍓

💖 Access and Printing of a HASH OF HASHES 💖

Getting at stuff and printing it all out:

# one element
$HoH{flintstones}{wife} = "wilma";

# another element
$HoH{simpsons}{lead} =~ s/(\w)/\u$1/;

# print the whole thing
foreach my $family ( keys %HoH ) {
    print "$family: { ";
    for my $role ( keys $HoH{$family}->%* ) {
        print "$role=$HoH{$family}{$role} ";
    }
    print "}\n";
}

# print the whole thing somewhat sorted
foreach my $family ( sort keys %HoH ) {
    print "$family: { ";
    for my $role ( sort keys $HoH{$family}->%* ) {
        print "$role=$HoH{$family}{$role} ";
    }
    print "}\n";
}

# print the whole thing sorted by number of members
foreach my $family ( sort { $HoH{$b}->%* <=> $HoH{$a}->%* } keys %HoH ) {
    print "$family: { ";
    for my $role ( sort keys $HoH{$family}->%* ) {
        print "$role=$HoH{$family}{$role} ";
    }
    print "}\n";
}

# establish a sort order (rank) for each role
my $i = 0;
my %rank;
for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i }

# now print the whole thing sorted by number of members
foreach my $family ( sort { $HoH{$b}->%* <=> $HoH{$a}->%* } keys %HoH ) {
    print "$family: { ";
    # and print these according to rank order
    for my $role ( sort { $rank{$a} <=> $rank{$b} }
                                              keys $HoH{$family}->%* )
    {
        print "$role=$HoH{$family}{$role} ";
    }
    print "}\n";
}

Little reminder bestie: keys $HoH{$family}->%* is the %* deref syntax, and $HoH{$b}->%* in numeric comparison gives you the count of keys. That's the vibe check for "who has more members". 🔥

---

🌈 MORE ELABORATE RECORDS 🌈

Okay now we're going full maximalist. Records where the fields are literally whatever you want — strings, arrays, hashes, code refs, filehandles, all chilling in one hash. It's giving ✨everything everywhere all at once✨.

🧚 Declaration of MORE ELABORATE RECORDS 🧚

Here's a sample showing how to create and use a record whose fields are of many different sorts:

my $rec = {
    TEXT      => $string,
    SEQUENCE  => [ @old_values ],
    LOOKUP    => { %some_table },
    THATCODE  => \&some_function,
    THISCODE  => sub { $_[0] ** $_[1] },
    HANDLE    => \*STDOUT,
};

print $rec->{TEXT};

print $rec->{SEQUENCE}[0];
my $last = pop $rec->{SEQUENCE}->@*;

print $rec->{LOOKUP}{"key"};
my ($first_k, $first_v) = each $rec->{LOOKUP}->%*;

my $answer = $rec->{THATCODE}->($arg);
$answer = $rec->{THISCODE}->($arg1, $arg2);

# careful of extra block braces on fh ref
print { $rec->{HANDLE} } "a string\n";

use FileHandle;
$rec->{HANDLE}->autoflush(1);
$rec->{HANDLE}->print(" a string\n");

Not me pointing out that print { $rec->{HANDLE} } needs those extra block braces when the filehandle is a ref. Don't @ me, it's just how it is. 😭

🎀 Declaration of a HASH OF COMPLEX RECORDS 🎀

Now stack them. A hash of complex records, full TV-guide realness:

my %TV = (
   flintstones => {
       series   => "flintstones",
       nights   => [ qw(monday thursday friday) ],
       members  => [
           { name => "fred",    role => "lead", age  => 36, },
           { name => "wilma",   role => "wife", age  => 31, },
           { name => "pebbles", role => "kid",  age  =>  4, },
       ],
   },

   jetsons     => {
       series   => "jetsons",
       nights   => [ qw(wednesday saturday) ],
       members  => [
           { name => "george",  role => "lead", age  => 41, },
           { name => "jane",    role => "wife", age  => 39, },
           { name => "elroy",   role => "kid",  age  =>  9, },
       ],
    },

   simpsons    => {
       series   => "simpsons",
       nights   => [ qw(monday) ],
       members  => [
           { name => "homer", role => "lead", age  => 34, },
           { name => "marge", role => "wife", age => 37, },
           { name => "bart",  role => "kid",  age  =>  11, },
       ],
    },
 );

Hash of hashes of arrays of hashes. Say that three times fast. Slay. 💅

⭐ Generation of a HASH OF COMPLEX RECORDS ⭐

# reading from file
# this is most easily done by having the file itself be
# in the raw data format as shown above.  perl is happy
# to parse complex data structures if declared as data, so
# sometimes it's easiest to do that

# here's a piece by piece build up
my $rec = {};
$rec->{series} = "flintstones";
$rec->{nights} = [ find_days() ];

my @members = ();
# assume this file in field=value syntax
while (<>) {
    my %fields = split /[\s=]+/;
    push @members, { %fields };
}
$rec->{members} = [ @members ];

# now remember the whole thing
$TV{ $rec->{series} } = $rec;

###########################################################
# now, you might want to make interesting extra fields that
# include pointers back into the same data structure so if
# change one piece, it changes everywhere, like for example
# if you wanted a {kids} field that was a reference
# to an array of the kids' records without having duplicate
# records and thus update problems.
###########################################################
foreach my $family (keys %TV) {
    my $rec = $TV{$family}; # temp pointer
    my @kids = ();
    for my $person ( $rec->{members}->@* ) {
        if ($person->{role} =~ /kid|son|daughter/) {
            push @kids, $person;
        }
    }
    # REMEMBER: $rec and $TV{$family} point to same data!!
    $rec->{kids} = [ @kids ];
}

# you copied the array, but the array itself contains pointers
# to uncopied objects. this means that if you make bart get
# older via

$TV{simpsons}{kids}[0]{age}++;

# then this would also change in
print $TV{simpsons}{members}[2]{age};

# because $TV{simpsons}{kids}[0] and $TV{simpsons}{members}[2]
# both point to the same underlying anonymous hash table

# print the whole thing
foreach my $family ( keys %TV ) {
    print "the $family";
    print " is on during $TV{$family}{nights}->@*\n";
    print "its members are:\n";
    for my $who ( $TV{$family}{members}->@* ) {
        print " $who->{name} ($who->{role}), age $who->{age}\n";
    }
    print "it turns out that $TV{$family}{lead} has ";
    print scalar ( $TV{$family}{kids}->@* ), " kids named ";
    print join (", ", map { $_->{name} } $TV{$family}{kids}->@* );
    print "\n";
}

🚨 Genuinely important, bestie, listen: $rec and $TV{$family} point to the same data. The array got copied but the array itself contains pointers to uncopied objects. So bump Bart's age through $TV{simpsons}{kids}[0]{age}++ and it also shows up at $TV{simpsons}{members}[2]{age}, because both of them point to the same underlying anonymous hash table. Aliasing. It's giving chaos. It's giving beware. 💀

🫶 Database Ties 🫶

You cannot easily tie a multilevel data structure (such as a hash of hashes) to a dbm file. The first problem is that all but GDBM and Berkeley DB have size limitations, but beyond that, you also have problems with how references are to be represented on disk. One experimental module that does partially attempt to address this need is the MLDBM module. Check your nearest CPAN site as described in perlmodlib for source code to MLDBM.

(Translation: it's messy, references-on-disk is a whole situation, MLDBM is the experimental lil module that partially tries.) 🧚

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🌸 SEE YOU IN THE SEE ALSO 🌸

• perlref
• perllol
• perldata
• perlobj

---

💜 AUTHOR 💜

Tom Christiansen <tchrist@perl.com> — the actual author. Respect. Crowned. 👑

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And that's the whole page, bestie!! You made it 🌈✨ this was perl v5.44.0, dated 2026-10-17, PERLDSC(1perl) — go forth and nest your data structures responsibly, and remember: copying an array copies the pointers, not the things they point at. Love u, bye!! 🫶💖🎀