...
sub pat_mc {
    our @common_letters;
    my $dummy = 0;
    our @wordsCopy = @words;
    my $reference = shift @wordsCopy;

    () = $reference =~ /(.)(?{        
       my $letter = $1; 
       my $position = $-[0]; 
       my $bolean = 1; 
       for ( @wordsCopy ) 
       { 
           if ( substr( $_, $position, 1 ) ne $letter )
           { 
               $bolean = 0; 
               last
           } 
       }
       $common_letters[ $position ] = $letter if ( $bolean ); 
       })/gx;
    return join '',
       map { $common_letters[ $_ ] || '-' }
       0 .. length( $reference ) - 1;
}