>perl -wMstrict -le
"my ($x, $y) = qw{abXcdYef pqXrsYtu};
 my @i_diffs = differing_positions($x, $y);
 print 'number of differences: ', scalar @i_diffs;
 print qq{differing positions: @i_diffs};
 sub differing_positions {
   my $i = 0;
   map   $_->[1],
   grep  $_->[0] ne qq{\x00},
   map   [ $_, $i++ ],
   split '',
   $_[0] ^ $_[1]
   ;
   }
"
number of differences: 6
differing positions: 0 1 3 4 6 7