In Regexes§

See primary documentation in context for Disable backtracking: :

The : metacharacter disables backtracking for the previous atom. Thus, / .*: a/ does not match " a" because the .* matches the entire string, leaving nothing for the a to match without backtracking.

In Variables§

See primary documentation in context for The : twigil

The : twigil declares a formal named parameter to a block or subroutine. Variables declared using this form are a type of placeholder variable too. Therefore the same things that apply to variables declared using the ^ twigil also apply here (with the exception that they are not positional and therefore not ordered using Unicode order). For instance:

say { $:add ?? $^a + $^b !! $^a - $^b }45 ) :!add
# OUTPUT: «-1␤»

See ^ for more details about placeholder variables.