use English '-no_match_vars';

my %appearances_of;
$appearances_of{$_}++ for map { split /\Q$SUBSCRIPT_SEPARATOR/ }
                              keys %hash;

##</code><code>##

        $hash{$1,$3} = "$holder";
# add these lines:
        $appearances_of{$1}++;
        $appearances_of{$3}++;