$ perl -E '
> $sa = q{abcdefg};
> $sb = q{abbdegg};
> $df = $sa ^ $sb;
> say q{Zero-based counting};
> 1 while $df =~ m{\x01(?{ say pos( $df ) - 1 })}g;
> say q{One-based counting as per OP example};
> 1 while $df =~ m{\x01(?{ say pos $df })}g;'
Zero-based counting
2
5
One-based counting as per OP example
3
6
$