Try searching CPAN for combinatorics-related modules, for example, Math::Combinatorics or Algorithm::Combinatorics. Also, it may be easier to help you if you tell us the purbose of generating such pairs of numbers (maybe it can be optimised too).

EDIT: example code

#!/usr/bin/perl

use warnings;
use strict;

use Math::Combinatorics;

my @n = (0..99);
my $cmb = Math::Combinatorics->new(count => 2, data => [@n]);

my %numbers;
$numbers{$_} = [] for @n;

while (my ($x,$y) = $cmb->next_combination) { # never returns $x == 99
+, so $numbers{99} will be always empty :(
    push $numbers{$x},$y;
}

print "($_,",$numbers{$_}->[int(rand($#{$numbers{$_}}+1))],")\n" for @
+n;
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