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

# Open the input.txt for reading in filehandle FILE
open FILE, "< input.txt";
my $totCols = 0;
my @hdrFields;
my @fields;
my @refs;

 # Read each line of input.txt from the filehandle
while (<FILE>) {
    # Remove the new line character from the line using chomp
    chomp;
    # If it's the first line being read then...
    if ($. == 1) {
        # Split the first line according to whitespaces and store the fields in array @hdrFields
        @hdrFields = split /\s+/;
        # Store the total number of columns in $totCols.
        # $#hdrFields refers to the last index number of attay @hdrFields
        $totCols = $#hdrFields + 1;
        
        # First 4 columns are fixed. So with 5th column as 1, iterate till the last column.
        # In the given example, there are 2 columns after col #4.
        # The below for-loop opens that many files for writing the output. In given example its 2.
        for (1 .. ($totCols - 4)) {
            # Reference to filehandles are stored in array @refs
            open ($refs[$_], "> $hdrFields[4 - 1 + $_].wig");
        }
        
        # After parsing the first line and opening the file handles for writing skip to the next line of file.
        # This will prevent the below code from being executed.
        next;
    }
    
    # For all lines after the header line, do the below:
    # First split the line according to whitespaces and store the fields in array @fields.
    @fields = split /\s+/;
    
    # Again, iterate through that many numbers equivalent to the number of columns after 4 (since first 4 columns are fixed)
    for (1 .. ($totCols - 4)) {
        # Retrieve the reference to file from array @refs
        my $fh = $refs[$_];
        # Print to corresponding file, first 4 fixed columns then the corresponding column. In this case, either col #5 or col #6.
        print $fh join("\t", @fields[1..3]) . "\t$fields[4 - 1 + $_]\n";
    }
}

# Finally close all the files that were opened for writing
for (1 .. ($totCols - 4)) {
    close $refs[$_];
}

# Finally close the input file that was opened for reading
close FILE;

[user@host ~]$ ls -l HEL25E.wig TRIP1.wig
-rw-r--r-- 1 user domain Users 226 Jun  7 16:32 HEL25E.wig
-rw-r--r-- 1 user domain Users 221 Jun  7 16:32 TRIP1.wig
[user@host ~]$