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

my $file = $ARGV[0] || 'file.txt';
open (INPUT, '<', $file) or die "Unable to open file for reading : $!";

my @values;
while ( <INPUT> ) {
    chomp;
    my @col = split /\t/;   # Assume basic tab delimited
    push @values, $col[3];  # Column 4 is what we want
}
my $total = 0;
$total += $_ for @values;
my $average = $total / @values;