Mnemonics

"negative", "number", "subtract"

-- usage is very intuitive

Used alone

- along with + coerces to type Num
- As an infix operator: subtracts numbers

Operators containing this character

- - pre and post decrement as in common parlance (which means use it like you would expect)

When used inside regexes

- at start of a character class changes the sense
- = the post-assignment mutant form of -

Other Uses

- before a flag on the commandline results in normal values in a MAIN subroutine
- - > used to specify return type in a Signature
- > and < - > introduces a pointy block
[*-1] finds the last entry of an array which is not special sugar but worth noting.
+ is a border intersection in POD
- can be used in identifiers, along with _ and '

Old, deprecated, or other language uses

- > is no longer the method operator as in perl5. That is now . like most other languages.
- delimits character class ranges like in most regular expression implementations. Perl6 uses the .. Range operator for that instead.