We could write the whole thing for you, but that's not what PerlMonks is about. What have you done to get started, and what part are you having trouble with?

My solution would differ depending on how large the input file is expected to be (or become). So as you work on providing more information, that's one element I would like to know.

Thank you for your comments. I just start learning perl and am not very used to script yet. So that is problem. I am not sure how to start this script yet. The file is very big like 500M. XF

What you need to accomplish this is regular expressions. Learning regular expressions can be a bit painful, but they're really incredibly powerful. As a sample, the code below might give a start point.

The relevant documentation is perlre

open ( my $input_fh, "<", $input_file );
open ( my $output_fh, ">", $output_file );
foreach my $line ( <$input_fh> )
{
  unless ( $line =~ m/\A\s*\Z/ )
  {
    $line =~ s/(GL\d{6}))\d+/$1/;
    print $output_fh $line;
  }
}
close ( $input_fh );
close ( $output_fh );
[download]

The essence is - first you test if a line is blank. Then you use a 'search and replace pattern' to trim any pattern starting GL, followed by 6 digits, to 6 digits.

How can I input the "input_file"? "$input_file" is a scalar but not a file. I run the following script:

#!/usr/local/bin/perl
use warnings;
open ( my $input_fh, "<", $genome );
open ( my $output_fh, ">", $output_file );
foreach my $line ( <$input_fh> )
{
  unless ( $line =~ m/\A\s*\Z/ )
  {
    $line =~ s/(GL\d{6})\d+/$1/;
    print $output_fh $line;
  }
}
close ( $input_fh );
close ( $output_fh );
[download]

and got the following messages:

Name "main::genome" used only once: possible typo at header.pl line 3. Name "main::output_file" used only once: possible typo at header.pl line 4. Use of uninitialized value $genome in open at header.pl line 3. Use of uninitialized value $output_file in open at header.pl line 4. readline() on closed filehandle $input_fh at header.pl line 5.

I think my way to open the file is wrong. Can you give me some hints? Thanks XF

foreach my $line ( <$input_fh> )

should be

while (my $line = <$input_fh> )

The first form is a glob, (but I don't know well enough to explain it to you).

The second line should work properly for reading your file.

Update: Yes Choroba is correct, it is not a glob - my mistake,

Ah, I didn't see your problem clearly. First, you should use strict; as well as use warnings;, which you did, in the header of your program. Then, you have to assign the name of your file to $genome.

my $genome = 'whateverthename';

(You must assign a name to your output file also)