A different approach might be to use the $/ variable ( see perlvar for details ) to read in each record and then test whether it has the 'pairs' field:

use warnings;
use strict;

{
        ## modify input record separator to read
        ## each FASTA record separately
    local $/ = '>';
        
        ## test each record, printing out if it has
        ## pair field
    while (<DATA>){
        print $_ if ($_ =~ /pair:\d+/);
    }
}
__DATA__
>CVSF43567.d1 bg|346280
CGTAGCTGATGCTGTGCTGTTGTGTCGGGGGGATATATATATATATATGGGGTCGTAGTCGTAGCGCTAG
TATGCTAGCAGCGTAGATGCTGATCGATGCTGATGCTGATCGTAGTCGTAGGCTAGTGCGATCGTAGTCG
TAGTCGATGCTGATGCGTAGCTGATGTGCTGCTGATGCTAGTCGTCGTAGCTGATGCATGCTGATCGTAG
TGCTCGATGCTAGTCGTAGTCGTAGTCGTAGCGACTGATGCGATCGTAGTCGGATGCTAGCACGTAGCTG
GCTCGATGCTGATGCTGAT
>CVSF10000.x1 bg|356789 pair:789860
ATGCGTAGCTGATGTGCTGCTGATGCTAGTCGTCGTAGCTGATGCATGCTGATCGTAGTGCTCGATGCTA
GTCGTAGTCGTAGTCGTAGCGACTGATGCGATCGTAGTCGGATGATGCTGACTGATGCTGATCTGTACGT
CGTAGCTGATGCATGCGCTAGTAGCT
>CVSF10000.y1 bg|356790 pair:789859
GCTAGTCGATGCTGATGCTGTAGCTAGCGTAGTCGTACGCGCGCGCGCGCGTTTTTTGTGACGTCGTAGT
CCGTAGCTGATGCGATGCTAGTGCTGTGTCAGCTGATGTCGTGTGTAGCTGATGCTGATCGTTCGTGTGT
CGATGCTGATGCTAGTCGTAGTGTAT
[download]

In this case I am simply testing for the presence of the 'pair' field using a regex (see perlre). Hope this helps!

Thank you for your help.

The code which I am trying to run - what is going wrong in there?? I am a beginner, and it would really help if I know how to modify my code to get the desired output??

Cheers!

It isn't really a case of right or wrong - you code is doing exactly what you are telling it to - sorting the header lines based on the descriptor field. As you will see all over the place, TIMTOWTDI (there is more than one way to do it) is a core part of Perl programming, so while your method could be made to work, it would be much more complicated than needed (you would need to buffer the squence associated with each header and print them out together, based on testing the header) and ultimately much harder to maintain etc...

Just try to work out (on paper if needs be) what steps you need to achieve, than code each one of those steps - shortcuts/better/different solutions may occur to you as you work, but the important thing is that the code is clear and well documented.

Maybe this doesn't answer your question, but really it is all about practice and experience, so the fact that you have made a good attempt shows that in time you'll be laughing at these sort of problems. Keep plugging away and if you get stuck again, come to the Monastery!

Just a something something...

use strict;
use warnings;

my $in = $ARGV[0];
my $p  = $ARGV[1];
my $s  = $ARGV[2];

open IN   , "<$in" or die $!;
open P_OUT, ">$p" or die $!; 
open S_OUT, ">$s" or die $!;

while (<IN>) {
    if (/^>/) {
        if (/pair:/) {
            select P_OUT;
        }
        else{
            select S_OUT;
        }
    }
    print;
}

close(IN);
close(P_OUT);
close(S_OUT);
[download]

Thanks for your help.

But all the above codes are printing just the header lines. I still am not able to print the data after the header lines for each record.

Am I am not seeing the obvious???

It seems to work for me.

Download my code and save it as a file named 847578.pl. Then download your data and save it in a file named fasta.txt. Then execute this at your command line:

perl ./847578.pl fasta.txt p.txt s.txt
[download]

I get 2 output files. This is the contents of file p.txt:

>CVSF10000.x1 bg|356789 pair:789860
ATGCGTAGCTGATGTGCTGCTGATGCTAGTCGTCGTAGCTGATGCATGCTGATCGTAGTGCTCGATGCTA
GTCGTAGTCGTAGTCGTAGCGACTGATGCGATCGTAGTCGGATGATGCTGACTGATGCTGATCTGTACGT
CGTAGCTGATGCATGCGCTAGTAGCT
>CVSF10000.y1 bg|356790 pair:789859
GCTAGTCGATGCTGATGCTGTAGCTAGCGTAGTCGTACGCGCGCGCGCGCGTTTTTTGTGACGTCGTAGT
CCGTAGCTGATGCGATGCTAGTGCTGTGTCAGCTGATGTCGTGTGTAGCTGATGCTGATCGTTCGTGTGT
CGATGCTGATGCTAGTCGTAGTGTAT
>CVSF10001.x1 bg|356791 pair:789862
AGTCGTAGTCGTAGCTGTAGCTGATGCTGTGTACGATGCTGATGCGATGCGTAGCGTAGCATCGATGCTA
CGACTAGTCGTAGTCGTC
>CVSF10001.y1 bg|356792 pair:789861
CGTAGCTGATGCTGATCGTAGTCGTAGTCGATGCGATGCTAGTCGTAGCTGTAGCTGATGCTGCGTGCTG
CAGTCGATGCTAGTCGATGCTGATCGTCTAGCAT
[download]

This is the contents of file s.txt:

>CVSF43565.d1 bg|346278
CAGACACACTTCTTTTAGTTGAGACACATGGAAAACATCATGTATGGCAGACAACTGTTCTGGGAGTTGG
ATCCGGGTAAGCAACGGGTCCACATATCTCCACAATCTCATAAGGGGCCAACATAGCGGGGGAGCTAACT
TGCCTTTGATTCCAAACCGTTGCACTCCTTTGGTCGGGGAAACTCGAAGGTACACATGATCACCAAGGTC
GAACTGCAGGGGTCTTCTCCGCTGGTCGGAGTAGCTCTTCTGTCAAGATTGGGCGGCCTTGAGATGTGCT
TGAATTACCTTCACTTGCTCTTCGGCTTCTGCCACTTAAGTCAGGGCCATAGACCTGTCTCTCCCCTGGG
CAGACACACTTCTTTTAGTTGAGACACATGGAAAACATCATGTATGGCAGACAACTGTTCTGGGAGTTGG
ATCCGGGTAAGC
>CVSF43566.d1 bg|346279
CAGACACACTTCTTTTAGTTGAGACACATGGAAAACATCATGTATGGCAGACAACTGTTCTGGGAGTTGG
ATCCGGGTAAGCCAGACACACTTCTTTTAGTTGAGACACATGGAAAACATCATGTATGGCAGACAACTGT
TCTGGGAGTTGGAATGCTAGTCGATCGCCAGACACACTTCTTTTAGTTGAGACACATGGAAAACATCATG
TTGGCAGACAACTGTTCTGGGAGTTGGATCCGGGTAAGCCAGACACACTTCTTTTAGTTGAGACACATGG
AAAACATCATGTATGGCAGACAACTGTTCTGGGAGTTGGATCCGGGTAAGC
>CVSF43567.d1 bg|346280
CGTAGCTGATGCTGTGCTGTTGTGTCGGGGGGATATATATATATATATGGGGTCGTAGTCGTAGCGCTAG
TATGCTAGCAGCGTAGATGCTGATCGATGCTGATGCTGATCGTAGTCGTAGGCTAGTGCGATCGTAGTCG
TAGTCGATGCTGATGCGTAGCTGATGTGCTGCTGATGCTAGTCGTCGTAGCTGATGCATGCTGATCGTAG
TGCTCGATGCTAGTCGTAGTCGTAGTCGTAGCGACTGATGCGATCGTAGTCGGATGCTAGCACGTAGCTG
GCTCGATGCTGATGCTGAT
[download]

The code below reads into two hashes the provided sequences depending on whether they have pair info or not at the descriptor portion of their corresponding records, writing each one of these hashes into a file is left as an exercise for the OP..

use strict;
use warnings;
use Bio::SeqIO;

my $file = "FastA.txt";
my $seq_obj = Bio::SeqIO->new(-file =>"<$file",-format=>'fasta',);

my %seqPair;
my %seq_NoPair;
while(my $seq = $seq_obj->next_seq){  #iterate through records
        if($seq->desc =~ /.*?pair.*?/i){
                $seqPair{$seq->id}=$seq->seq;
                }else{
                        $seq_NoPair{$seq->id}=$seq->seq;
                        }
        }
        
#use Data::Dump qw(pp);
#print pp \%seqPair;
#print pp \%seq_NoPair;
[download]