Perl 6
WITCH underscore: Revision 3
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
_ is the only symbol allowed within identifier names along with numbers and letters
_ 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
