Mnemonics

"Understood"

-- think of like a grammatical understood subject
-- also, a solid version of a space

Special variables

$_ is the current topic, receiving iterators inside loops, etc.
@_ contains ordered parameters inside perl5ish subroutines or pointy blocks,
%_ is like @_ but contains named parameters.

Other uses

_ along with - and ', the only symbols allowed within identifier names
_ can also be included inside literal numbers to separate digits for purely cosmetic purposes

Old, deprecated, or other language uses

For a while, _ was considered to replace . as the string concatinator, but ~ was chosen for that instead