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

# Predeclare variables.
my %test;
my @fileData;
my @sorted;

# Initialize the variable that will hold the data to be changed.
my $change = "this is cool";

# Open the file
open(TEST, "test.txt") || die "File couldn't be opened for reading: $!";

# Create a hash
while (<TEST>) {
my ($key,@fileData) = split /\|/;
chomp @fileData;
$test{$key} = \@fileData;
}

# Change the data
$test{mykey}->[1] = "$change";

# Place all the hash data into an array of pipe-seperated values for sorting.
foreach my $key (keys %test) {
	push (@sorted, "$key|$test{$key}->[0]|$test{$key}->[1]|$test{$key}->[2]|$test{$key}->[3]");
}

# Sort the data by the number in the last part of the array.
@sorted = map $_->[1],
	sort { $a->[0] <=> $b->[0] }
		map [ substr($_,rindex($_,'|')+1), $_ ],
		@sorted;

# Put the line breaks back in.
for (@sorted) {
$_ = "$_\n";
}

open(TEST, ">test.txt") || die "File couldn't be opened for writing: $!";
print TEST @sorted;
close(TEST)