From language/variables

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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 }( 4, 5 ) :!add
# OUTPUT: «-1␤»

See ^ for more details about placeholder variables.