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 the stop condition (including items that first matched the start condition).

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

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

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