in reply to $a <=> $b

roboticus' answer is correct. The documentation for the special variables $a and $b is in perlvar.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

In Section Seekers of Perl Wisdom