Documentation for infix fff assembled from the following pages:

Language documentation: Operators §

From Operators

(Operators) infix fff §

sub infix:<fff>(Mu $a, Mu $b)

Performs a sed-like flipflop operation, wherein it returns False until the left argument smartmatches against $_, then returns True until the right argument smartmatches against $_.

Works similarly to ff, except that it only tries one argument per invocation. That is, if $_ smartmatches the left argument, fff will not then try to match that same $_ against the right argument.

for <AB C D B E F> {
    say $_ if /A/ fff /B/;         # OUTPUT: «AB␤C␤D␤B␤» 
}

The non-sed-like flipflop (which after successfully matching the left argument against $_ will try that same $_ against the right argument and act accordingly). See ff.

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