for (my $i = 0; $i < scalar(@mainseq); ++$i)
{
$rvscomp = reverse $mainseq[$i][1];
$rvscomp =~ tr/ACGT/TGCA/; 
$fwd100 = substr($mainseq[$i][1],0,100);
$rvs100 = substr($rvscomp,0,100);
print "$mainseq[$i][0]_forward_first_100\n$fwd100\n$mainseq[$i][0]_reversecomp_first_100\n$rvs100\n";
}