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: reset sysconfdir to etc

Keep sysconfdir set to etc to avoid losing existing user configuration. sysconfdir was changed in ca09a39.
irssi: update 0.8.17_2 bottle.