From language/operators

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sub infix:<^ff>(Mu $a, Mu $b)

Works like ff, except it does not return True for items matching the start condition (including items also matching the stop condition).

A comparison:

my @list = <A B C>;
say $_ if /A/ ff /C/ for @list;    # OUTPUT: «A␤B␤C␤» 
say $_ if /A/ ^ff /C/ for @list;   # OUTPUT: «B␤C␤»

The sed-like version can be found in ^fff.

This operator cannot be overloaded, as it's handled specially by the compiler.