Mnemonics

"hard quote", "tick"

• quote without much interpolation
• and a couple other minor uses

Used as a brace pair or quote pair

'' in a bare string literal does minimal interpolation like q//
'' when used with a quote introducer like q, qq, qw is just a generic quote

When used inside regexes

'' chunks multiple gliphs into a single literal as well as escaping metacharacters.

Other Uses

'' around a method name in a call refers to the prefix operator in method postfix form
' can be used as a symbol in an identifier

Old, deprecated, or other language uses

' as a namespace separator as in perl5 is always done with :: in perl6