Sorry, you are misinformed. That hasn't been true for a long time, see http://p3rl.org/Kavorka::Manual::Signatures
Other languages would kill for this kind of beyond-complete maximum overkill feature set.
> This page gives a good overview of the situation: https://perl101.org/subroutines.html
No, it doesn't. That page is not wrong, but incomplete and teaches the wrong stuff, wouldn't recommend. Instead see http://perl-tutorial.org/ for recommended tutorials.