That text is somewhat misleading - and note that it does not mention the UTF8 flag, indeed it explicitly says you "shouldn't worry" about what the internal format is.

Perl does not distinguish between text strings and binary strings, but programmers may do so when deciding how to interpret the contents of a string. Thus they may interpret input received from an outside source as a sequence of utf8 octets, and decide that they need to decode it to get the desired sequence of characters. The string "\x{c3}\x{9f}" is a sequence of two characters; if the programmer interprets it as a sequence of utf8 octets, they might choose to decode it to get the string "\x{df}". Those are two different strings.

However the string "\x{c3}\x{9f}" has two different possible internal representations, one with and one without the UTF8 flag enabled. It is the same string - the same sequence of characters - regardless of the internal representation. The same is true of the two different possible internal representations of "\x{df}".

Any time the abstraction leaks out - any time you need to care about which internal representation is being used for the string - that's an example of the Unicode bug.

These days I'm working with binary data in Perl, so I'm a bit skeptical about such claims. Consider:

In $upper_a, the UTF8 Flag is off. All character functions of Perl work on this variable, and that's not only for backwards compatibility. I'd rather call it a text string than a binary string.

If I create a character with $upper_a = "\N{U+41}", then the result has the UTF8 flag on. It behaves in no way different from $upper_a created before.

Binary strings in Perl need careful treatment. Strings where the UTF8 flag is off qualify as binary strings - but these can also be used as character strings. Applying character functions to a binary string might switch the UTF8 flag on. The flag is contagious, one term where the flag is on (even when it is on an character from the ASCII set) is sufficient to have the result carrying the flag. Whether that's a problem depends on whether each position (to avoid the terms "byte" and "character" for the moment) holds a value of less than 256.

If I create an 'ä' as \N{U+E4}, then the UTF8 flag is on. Perl's internal PV is "\303\244" (0xC3A4). But still, it can be treated like a binary 0xE4: It matches /\xE4/, and it can be printed without a "wide character" warning as a single byte.

One major problem is - as usual -that various perldocs were written by different authors each using another set of terminology.
• binary, byte, octet, etc for strings without flag
• character, text, etc for strings with flag
• sometimes other words like "stream" are used to abstract "string".
• the flag is named utf8-flag even if the internal format isn't 100% UTF8 and we still need to decode utf8 encoded strings.
• (update: mind also UTF-8 vs. utf8 vs. UTF8)
Furthermore is Perl - predominantly - operator dominated, contrary to type dominations in other languages.

We can use a bit operation on strings and so called the "utf8-flag" shouldn't matter at all, because the text-interpretation of the data is irrelevant.

Much of the confusion in this thread is due to terminology.

And when I read canonical sources like the following it's messy:

• perluniintro
• perlunitut
• perlunicode
• Encode
• utf8
• perlunifaq
Those perldocs are only the tip of the iceberg, because various other operator specific docs build on their terminology (or not)

I wonder if the authors would be able to talk to each other.

Cheers Rolf
_{(addicted to the 𐍀𐌴𐍂𐌻 Programming Language :)
Wikisyntax for the Monastery}

Indeed!

As you know, I've written a fair bit about this. I use these terms to be as unambiguous as possible:

---

Terms describing what a string represents (unrelated to storage format):

• String of decoded text aka string of Unicode Code Points
• String of bytes, such as a string of encoded text

For example, /\w/ only works when applied to a string of decoded text.

For example, pack returns a string of bytes.

"String of bytes" can still confuse people, so I try to avoid that one by being more specific (e.g. by using "encoded text" instead of "bytes").

---

Terms describing the internal storage format (unrelated to semantics):

• Downgraded string
• Upgraded string