Is it me, or did everyone miss the simplest approach?

sub localize_and_call_fn {
    my ( $locals, $fn, @args ) = @_;
    if( @$locals ) {
        my ( $sym, @local ) = @$locals;
        local *$sym = sub { "changed" };
        @local ? localize_and_call_fn( \@local, $fn, @args ) : $fn->( 
+@args );
    }
}
[download]

Makeshifts last the longest.