https://metacpan.org/release/LWALL/perl5.002b3/source/pod/perlre.pod does not contain the word POSIX, but the current perlre does. So as a first guess, Perl 5.002b3 did not support POSIX REs.

https://metacpan.org/release/GSAR/perl5.005-beta2/source/pod/perlre.pod also does not contain the word POSIX. https://metacpan.org/release/GSAR/perl-5.6.0/source/pod/perlre.pod does. So, my second guess is POSIX REs were introduced in 5.6.0. And indeed, https://metacpan.org/release/GSAR/perl-5.6.0/view/pod/perldelta.pod#POSIX-character-class-syntax-%5B:-:%5D-supported lists POSIX character class syntax as new feature for 5.6.0.

Alexander