#!/usr/bin/perl
use strict;
use warnings;

my $filename = 'counts.txt';

open (my $fh, '+<', $filename)
    or die "Could not open '$filename' $!";

my $count = "";

while (<$fh>) {
    chomp;
    if ($_ =~ /^>hsa/) {
	$count .= length $_;
        $count .= " ";
    }
}

print $fh $count . "\n";
close $fh
    or die "Could not close '$filename' $!";

__END__

>hsa_circ_0000001|chr1:1080738-1080845-|None|None
ATGGGGTTGGGTCAGCCGTGCGGTCAGGTCAGGTCGGCCATGAGGTCAGGTGGGGTCGGCCATGAAGGTGGTGGGGGTCATGAGGTCACAAGGGGGTCGGCCATGTG
>hsa_circ_0000002|chr1:1158623-1159348-|NM_016176|SDF4
GGTGGATGTGAACACTGACCGGAAGATCAGTGCCAAGGAGATGCAGCGCTGGATCATGGAGAAGACGGCCGAGCACTTCCAGGAGGCCATGGAGGAGAGCAAGACACACTTCCGCGCCGTGGACCCTGACGGGGACGGTCACGTGTCTTGGGACGAGTATAAGGTGAAGTTTTTGGCGAGTAAAGGCCATAGCGAGAAGGAGGTTGCCGACGCCATCAGGCTCAACGAGGAACTCAAAGTGGATGAGGAAA
49 54