A brute force approach, as you indicated, would iterate across the same array in a nested fashion. You would perhaps use code like this (untested):

use strict;
use warnings;
my $fh;
unless (open($fh,"</path/list.txt")) {
  die "Cannot open file: $!\n";
}
my @number_array = ();
while (<$fh>) {
  push @number_array, $_;
}
for (my $i=0;$i<$#number_array;$i++) {
  for (my $j=$i+1;$j<=$#number_array;$j++) {
    print $number_array[$i], "\t", $number_array[$j], "\n";
  }
}
[download]

Update: fixed error in outer loop -- only needs to iterate up to (n-1)th array index.