sub compatible {
    my ($s1, $s2) = @_;
    my($c1, $c2, $d);
    {
        $s1 =~ /(.)/gs or return 1;
        $c1 = $1;
        $s2 =~ /(.)/gs or return 1;
        $c2 = $1;
        if ($c1 eq "_") {
                if ($c2 eq "_")
                        { }
                else
                        { vec($d, ord $c2, 1)++ and return; }
        } else {
                if ($c2 eq "_")
                        { vec($d, ord $c1, 1)++ and return; }
                else
                        { $c1 ne $c2 and return; vec($d, ord $c2, 1)++ and return; }
        }
    redo;
    }
}

printf "%s v %s ? %s\n", @$_, compatible( @$_ ) || 0
        for [ qw[ _8__3__19 48____7__ ] ],  # compat
                [ qw[ _8__3__19 4_2___7__ ] ],  # compat
                [ qw[ _8__3__19 4_8___7__ ] ],  # clash
                [ qw[ __8_3__19 48____7__ ] ],  # clash
                [ qw[ __8_3__19 84____7__ ] ],  # clash
                [ qw[ _8__3__19 48_____7_ ] ];  # clash