that was too tough for me, so i asked a friend of mine (who is a real perl wizard but not very talkative). my own solution was hash-based too. he gave me this:

use strict;
use warnings;

print &compatible ("_8__3__19", "48____7__"); #c 
print &compatible ("_8__3__19", "4_8___7__"); #i 
print &compatible ("_8__3__19", "48_____7_"); #i


sub compatible
{
    my @s=($_[0], $_[1]);

    my @d;

    for( 0,1 )
    {
      for( $d[$_]=$s[$_] )
      {
        tr/0-9/\0/c;  #assuming the placeholder is not \0
        tr/\0/\377/c;
      }
    }
    
    my $m="$d[0]" & "$d[1]";
    
    for( 0,1 )
    {
        $d[$_] = "$m" & "$s[$_]";
    }
    
    my $compatible;
    if( $compatible = $d[0] eq $d[1] )
    {
        for( 0,1 )
        {
        $d[$_] = {};
        ${$d[$_]}{$1} .= "$-[0] " while $s[$_] =~ /(\d)/g;
        }

        $compatible &= !grep$d[1]->{$_} && $d[1]->{$_} ne $d[0]->{$_},
+keys %{$d[0]};
    }
    
    return $compatible ? 1 : 0;;
}
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i canīt say i understand it fully but normally you can rely on ozo