use re 'eval';

my $re = qr/((.)\2+)/;

my ($maxlen, $maxchr, $maxloc);
$_="aabccccdddeffff";

1 while m/$re(?{
    $maxlen = length($^N),
    $maxchr = substr($^N,0,1),
    $maxloc = pos() - $maxlen
        if length($^N) > $maxlen;
})/g;

print $maxchr, ' x ', $maxlen, ' at ', $maxloc, $/;

__END__
c x 4 at 3