I stripped this for me down to

$regexp="a{2}";
$_="aaaa";
do {
  push @a, $1 if ( m/^($regexp)/ ) 
} while ( s/^.// ) ;
print join ( "-", @a );
[download]

The main difference is, that I match the pattern at the beginning of the string while you match it on the substr

Thanks for the input.