Here's another way to do it, if I correctly understand what you want to do. You want to find an index $i for which $a[$i] eq $a and $b[$i] ne $b (that is, your condition holds at the same index). So why are you searching the array elements? It's better to search for an appropriate index!

@indices = grep
             { $a[$_] eq $a and $b[$_] ne $b }
               (0..$#a);
[download]