irssi: restore perl

Replaces perl functionality; adds fix to avoid picking up perl destinations outside of Homebrew's prefix if a user has set PERL_MM_OPT like CPAN suggests.

irssi's configure script uses perl's ExtUtils::MakeMaker to finesse perl library paths. The `PERL_MM_OPT` environment variable influences MakeMaker's output. CPAN recommends adding a `INSTALL_BASE` option to `PERL_MM_OPT` in shell initialization scripts. When this is set, it overrides the `LIB` option that irssi's configure passes in, so irssi ends up using non-Homebrew paths. Unsetting `PERL_MM_OPT` in the formula prevents CPAN's configuration from affecting our build.

Discussion in #34685; supercedes #34770; partially reverts ca09a39 from #34692.