irc/irssi: Unbreak when PERL=OFF

In r556069 the port changed to build with meson. This requires perl to build
regardles of the option PERL that allows perl scripts to be run in irssi.

While here clarify pkg-message (sent by daniel.engberg.lists@pyret.net)

PR: 251371 251360
Submitted by: dor.bsd@xm0.uk (maintainer)
Reported by: m.bueker@berlin.de