What does “0 but true” mean in Perl?

It’s normal behaviour of the language. Quoting the perlsyn manpage:

The number 0, the strings '0' and "", the empty list (), and undef
are all false in a boolean context. All other values are true. Negation
of a true value by ! or not returns a special false value.
When evaluated as a string it is treated as "", but as a number, it is
treated as 0.

Because of this, there needs to be a way to return 0 from a system call that expects to return 0 as a (successful) return value, and leave a way to signal a failure case by actually returning a false value. "0 but true" serves that purpose.