use v5.10;

sub longest_rep{
    $_[0] =~ /(?=(.+)\1)(?!(??{ '.+?(..{' . length($^N) . ',})\1' }))/;
}

say longest_rep '01011010';
say longest_rep '010110101000110001';
say longest_rep '2323191919191919';
say longest_rep '22010110100';