Because they're using regex101 to easily test the semantics of different regex engines and Perl isn't available on regex101. PCRE is though, which is a decent approximation. And indeed, Perl and PCRE behave the same for this particular case.
I dunno. Maybe because nobody has contributed it? Maybe because Perl isn't as widely used as it once was? Maybe because it's hard to compile Perl to WASM? Maybe some other reason?