PERLUNIFAQ(1)

💖 NAME 💖

perlunifaq — the Perl Unicode FAQ 🦋 (yes queen, it's giving "I have questions and I need answers")

---

🎀 Q AND A 🎀

okay so this is a whole list of questions and answers about Unicode in Perl, and the vibe is: you should read this after perlunitut. like, it's the sequel, the second book in the duology, the encore 🫶

🧚 perlunitut isn't really a Unicode tutorial, is it?

No bestie, and ngl this isn't really a Unicode FAQ either 😭

Here's the tea: Perl has an abstracted interface for all the character encodings it supports, so lowkey this is actually a generic "Encode" tutorial and a generic "Encode" FAQ. But so many people think Unicode is special and magical ✨ and I didn't want to disappoint them, so I just called the whole thing a Unicode tutorial. Marketing, babes 💅

🌈 What character encodings does Perl support?

To find out which character encodings your Perl supports, run this:

perl -MEncode -le "print for Encode->encodings(':all')"

👑 Which version of perl should I use?

Well, if you can, upgrade to the most recent one — but for sure you want 5.8.1 or newer. The tutorial and the FAQ both assume you're on the latest release, periodt.

You should also check your modules and upgrade them if necessary. For example, HTML::Entities requires version >= 1.32 to function correctly, even though the changelog is suspiciously silent about this 🙈

🖼️ What about binary data, like images?

Well, apart from a bare binmode $fh, you shouldn't treat them specially. (The binmode is needed because otherwise Perl may convert line endings on Win32 systems. not cute, we don't want that 💀)

Be careful though bestie — never combine text strings with binary strings. If you need text in a binary stream, encode your text strings first using the appropriate encoding, then join them with binary strings. See also: "What if I don't encode?" below 👀

🌸 When should I decode or encode?

Whenever you're communicating text with anything that is external to your perl process — like a database, a text file, a socket, or another program. Even if the thing you're communicating with is also written in Perl. no exceptions, fr fr 💅

😭 What if I don't decode?

Whenever your encoded, binary string gets used together with a text string, Perl will assume that your binary string was encoded with ISO-8859-1, also known as latin-1. And if it wasn't latin-1? Then your data gets unpleasantly converted 😬

For example, if it was UTF-8, the individual bytes of multibyte characters are seen as separate characters, and then converted to UTF-8 again. Such double encoding can be compared to double HTML encoding (>), or double URI encoding (%253E). it's giving chaos honestly.

This silent implicit decoding is known as "upgrading". That may sound positive, but bestie, it's best to avoid it. it's a trap 🪤

💀 What if I don't encode?

It depends on what you output and how you output it. two whole scenarios, buckle up:

Output via a filehandle

• If the string's characters are all code point 255 or lower, Perl outputs bytes that match those code points. This is what happens with encoded strings. It can also, though, happen with unencoded strings that happen to be all code point 255 or lower.
• Otherwise, Perl outputs the string encoded as UTF-8. This only happens with strings you neglected to encode. And since that should not happen, Perl also throws a "wide character" warning in this case. it's calling you out 📢

Other output mechanisms (e.g., exec, chdir, ..)

Your text string will be sent using the bytes in Perl's internal format.

Because the internal format is often UTF-8, these bugs are hard to spot — because UTF-8 is usually the encoding you wanted! 😭 But don't be lazy, and do not use the fact that Perl's internal format is UTF-8 to your advantage. Encode explicitly to avoid weird bugs, and to show to maintenance programmers that you thought this through. main character energy: thinking ahead 🧠✨

🦋 Is there a way to automatically decode or encode?

If all data that comes from a certain handle is encoded in exactly the same way, you can tell the PerlIO system to automatically decode everything, using the encoding layer. And if you do this, you can't accidentally forget to decode or encode anymore, on things that use the layered handle. set it and forget it, iconic 💅

You can provide this layer when opening the file:

open my $fh, '>:encoding(UTF-8)', $filename;  # auto encoding on write
open my $fh, '<:encoding(UTF-8)', $filename;  # auto decoding on read

Or if you already have an open filehandle:

binmode $fh, ':encoding(UTF-8)';

Some database drivers for DBI can also automatically encode and decode, but that is sometimes limited to the UTF-8 encoding. so close 😔

🔍 What if I don't know which encoding was used?

Do whatever you can to find out, and if you have to: guess. (And don't forget to document your guess with a comment! future you will thank you 🫶)

You could open the document in a web browser, and change the character set or character encoding until you can visually confirm that all characters look the way they should. squint at it, vibe check it 👀

There is no way to reliably detect the encoding automatically, so if people keep sending you data without charset indication, you may have to educate them. be the teacher you wish to see in the world 🧚

✨ Can I use Unicode in my Perl sources?

Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the use utf8 pragma.

use utf8;

This doesn't do anything to your input, or to your output. It only influences the way your sources are read. You can use Unicode in string literals, in identifiers (but they still have to be "word characters" according to \w), and even in custom delimiters. slay 💅

🙈 Data::Dumper doesn't restore the UTF8 flag; is it broken?

No — Data::Dumper's Unicode abilities are exactly as they should be. There have been some complaints that it should restore the UTF8 flag when the data is read again with eval. However, you should really not look at the flag, and nothing indicates that Data::Dumper should break this rule. it's innocent, your honor ⚖️

Here's what actually happens: when Perl reads in a string literal, it sticks to 8 bit encoding as long as it can. (But perhaps originally it was internally encoded as UTF-8, when you dumped it.) When it has to give that up because other characters are added to the text string, it silently upgrades the string to UTF-8. sneaky little upgrade 😌

If you properly encode your strings for output, none of this is of your concern, and you can just eval dumped data as always. problem? solved. 💖

🌈 Why do regex character classes sometimes match only in the ASCII range?

Starting in Perl 5.14 (and partially in Perl 5.12), just put a use feature 'unicode_strings' near the beginning of your program. Within its lexical scope you shouldn't have this problem. It also is automatically enabled under use feature ':5.12' or use v5.12 or using -E on the command line for Perl 5.12 or higher. one line, fixes everything, we love to see it ✨

The rationale for requiring this is to not break older programs that rely on the way things worked before Unicode came along. Those older programs knew only about the ASCII character set, and so may not work properly for additional characters. When a string is encoded in UTF-8, Perl assumes that the program is prepared to deal with Unicode — but when the string isn't, Perl assumes that only ASCII is wanted, and so those characters that are not ASCII characters aren't recognized as to what they would be in Unicode. use feature 'unicode_strings' tells Perl to treat all characters as Unicode, whether the string is encoded in UTF-8 or not, thus avoiding the problem. 💅

However, on earlier Perls, or if you pass strings to subroutines outside the feature's scope, you can force Unicode rules by changing the encoding to UTF-8 by doing utf8::upgrade($string). This can be used safely on any string, as it checks and does not change strings that have already been upgraded. safe, checked, no notes 🫶

For a more detailed discussion, see Unicode::Semantics on CPAN.

😤 Why do some characters not uppercase or lowercase correctly?

See the answer to the previous question. literally just scroll up bestie ⬆️

🕵️ How can I determine if a string is a text string or a binary string?

You can't. 😭 Some people use the UTF8 flag for this, but that's misuse, and it makes well behaved modules like Data::Dumper look bad. The flag is useless for this purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is used to store the string.

This is something you, the programmer, have to keep track of; sorry. You could consider adopting a kind of "Hungarian notation" to help with this. yeah it's a whole thing 💀

🔁 How do I convert from encoding FOO to encoding BAR?

By first converting the FOO-encoded byte string to a text string, and then the text string to a BAR-encoded byte string:

my $text_string = decode('FOO', $foo_string);
my $bar_string  = encode('BAR', $text_string);

or by skipping the text string part entirely, and going directly from one binary encoding to the other:

use Encode qw(from_to);
from_to($string, 'FOO', 'BAR');  # changes contents of $string

or by letting automatic decoding and encoding do all the work:

open my $foofh, '<:encoding(FOO)', 'example.foo.txt';
open my $barfh, '>:encoding(BAR)', 'example.bar.txt';
print { $barfh } $_ while <$foofh>;

three ways, pick your fighter 🥊

🎀 What are "decode_utf8" and "encode_utf8"?

These are alternate syntaxes for decode('utf8', ...) and encode('utf8', ...).

Do not use these functions for data exchange. Instead use decode('UTF-8', ...) and encode('UTF-8', ...); see "What's the difference between UTF-8 and utf8?" below. 👀

🐘 What is a "wide character"?

This is a term used for characters occupying more than one byte.

The Perl warning "Wide character in ..." is caused by such a character. With no specified encoding layer, Perl tries to fit things into a single byte. When it can't, it emits this warning (if warnings are enabled), and uses UTF-8 encoded data instead. it tried its best okay 😭

To avoid this warning and to avoid having different output encodings in a single stream, always specify an encoding explicitly, for example with a PerlIO layer:

binmode STDOUT, ":encoding(UTF-8)";

---

💅 What is "the UTF8 flag"? 💅

Okay bestie, listen. Unless you are literally hacking the internals or debugging some cursed weirdness, do not think about the UTF8 flag at all. Like, at all. Which means you very probably should not be using is_utf8, _utf8_on or _utf8_off either. Put them down. Step away. 🫶

The UTF8 flag, also called SvUTF8, is an internal flag that says "hey, the current internal representation is UTF-8". No flag? Then it's assumed to be ISO-8859-1. Perl flips between these automatically, no drama, no notes. (Actually Perl usually assumes the representation is ASCII — go peek at "Why do regex character classes sometimes match only in the ASCII range?" above, iykyk.)

Here's the tea: one of Perl's internal formats just happens to be UTF-8. And unfortunately Perl cannot keep a secret, so everyone knows about it, and that is the source of SO much confusion. 😭 The move is to pretend the internal format is some totally unknown encoding and that you always have to encode and decode explicitly. Delulu? No, it's strategy. ✨

---

🚫 What about the use bytes pragma? 🚫

Don't use it. Periodt.

It makes no sense to deal with bytes in a text string, and it makes no sense to deal with characters in a byte string. Do the proper conversions (by decoding/encoding) and everything works out beautifully: you get character counts for decoded data, and byte counts for encoded data. Chef's kiss. 👑

use bytes is usually a failed attempt to do something useful. Just forget about it. It's giving ex you don't text back. 💀

---

🙈 What about the use encoding pragma? 🙈

Also don't. Not the vibe.

The problem: it assumes the programmer's environment and the user's environment use the same encoding. It'll use the same encoding for the source code and for STDIN and STDOUT. But when a program gets copied to another machine, the source code does not change — while the STDIO environment totally might. And then everything breaks. 😵

So if you need non-ASCII characters in your source code, make it a UTF-8 encoded file and use utf8.

And if you need to set the encoding for STDIN, STDOUT, and STDERR — like, based on the user's locale — then use open. That's the one. 🌸

---

🦋 What is the difference between :encoding and :utf8? 🦋

So here's why this even comes up: because UTF-8 is one of Perl's internal formats, you can often just skip the encoding or decoding step and manipulate the UTF8 flag directly. Tempting, right? 🙈

Instead of :encoding(UTF-8), you can simply use :utf8, which skips the encoding step if the data was already represented as UTF8 internally. This is widely accepted as good behavior when you're writing, but it can be dangerous when reading, because it causes internal inconsistency when you have invalid byte sequences.

Using :utf8 for input can sometimes result in security breaches, so please use :encoding(UTF-8) instead. 🚨

Also, instead of decode and encode, you could use _utf8_on and _utf8_off — but that's considered bad style. Especially _utf8_on, which can be dangerous for the exact same reason :utf8 can. So, no. 💅

There are some shortcuts for oneliners though; go see -C in perlrun.

---

⭐ What's the difference between UTF-8 and utf8? ⭐

UTF-8 is the official standard. utf8 is Perl's way of being liberal in what it accepts. Very "come as you are", very accepting, maybe too accepting. 🧚

So: if you have to communicate with things that aren't so liberal, you may want to consider using UTF-8. And if you have to communicate with things that are too liberal, you may have to use utf8. The full explanation lives in "UTF-8 vs. utf8 vs. UTF8" in Encode.

UTF-8 is internally known as utf-8-strict. The tutorial uses UTF-8 consistently, even where utf8 is actually used internally, because the distinction can be hard to make and is mostly irrelevant.

Example time: utf8 can be used for code points that don't exist in Unicode, like 9999999, but if you encode that to UTF-8, you get a substitution character (by default — see "Handling Malformed Data" in Encode for more ways of dealing with this).

Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not some other encoding.) 💀

---

🌈 I lost track; what encoding is the internal format really? 🌈

Honestly? It's good that you lost track, because you shouldn't depend on the internal format being any specific encoding. Main character behavior: unbothered, unbothered. ✨

But since you asked: by default, the internal format is either ISO-8859-1 (latin-1), or utf8, depending on the history of the string. On EBCDIC platforms, this may be different even.

Perl knows how it stored the string internally, and will use that knowledge when you encode. In other words: don't try to find out what the internal encoding for a certain string is — just encode it into the encoding that you want. That's it, that's the whole thing. 🎀

---

👑 AUTHOR 👑

Juerd Waalboer <#####@juerd.nl>

---

🫶 SEE ALSO 🫶

• perlunicode
• perluniintro
• Encode

---

and that's the whole page bestie, we ate and left no crumbs 💖 this was perl v5.42.3, dated 2026-08-03, filed under PERLUNIFAQ(1) — now go encode something responsibly, mwah 💅🌸