You can install it from <https://scripts.irssi.org>:
mkdir -p ~/.irssi/scripts/autorun
- wget https://scripts.irssi.org/scripts/cap_sasl.pl -O ~/.irssi/scripts/cap_sasl.pl
- ln -sf ../cap_sasl.pl ~/.irssi/scripts/autorun/
+ cd ~/.irssi/scripts/autorun
+ wget https://scripts.irssi.org/scripts/cap_sasl.pl -O ../cap_sasl.pl
+ ln -sf ../cap_sasl.pl .
Now load and configure it inside Irssi: