Irssi looks in a hard-coded @INC that is not the real @INC from the current Perl binary. As such, a freshly installed irssi will not function with a freshly installed Perl 5.8.4. The solution is to rebuild the irssi package and modify the line deleting the Perl pod file, as it is now a longer directory name than i686-linux, has threaded and crap in it. This PKGBUILD works fine:

# $Id: PKGBUILD,v 1.17 2004/03/20 09:29:36 judd Exp $
# Maintainer: dorphell <dorphell@archlinux.org>
pkgname=irssi
pkgver=0.8.9
pkgrel=3
pkgdesc="Modular text mode IRC client with Perl scripting"
url="http://irssi.org/"
depends=('glib2' 'openssl' 'gpm')
source=(http://irssi.org/files/$pkgname-$pkgver.tar.bz2)
md5sums=('6610ee0e27922f447e40828cf7dee507')

build() {
cd $startdir/src/$pkgname-$pkgver
./configure --prefix=/usr
make || return 1
make DESTDIR=$startdir/pkg install
rm $startdir/pkg/usr/lib/perl5/*/*/perllocal.pod
}