From language/operators

See Original text in context

sub infix:<^ff^>(Mu $a, Mu $b)

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

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␤»

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

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