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

my $usage = qq{$0 input_fastq trim_length};
die "$usage\n" if scalar @ARGV != 2;
my ($fastq, $trim_length) = @ARGV;

open(FASTQ, $fastq) or die "Can't open $fastq\n";
while (my $readid = <FASTQ>) {
        chomp $readid;
        chomp (my $sequence  = <FASTQ>);
        chomp (my $comment   = <FASTQ>);
        chomp (my $quality   = <FASTQ>);

        my $sub_seq      = length $sequence < $trim_length ? $sequence : substr $sequence, 0, $trim_length;
        my $sub_quality  = length $sequence < $trim_length ? $quality  : substr $quality,  0, $trim_length;
        print qq{$readid\n$sub_seq\n$comment\n$sub_quality\n};

}
close FASTQ;