#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my %hash;

while (<DATA>) {
    next unless /[^[:space:]]/; # make sure there's something on the line

    my ($key,$value)    = split;
    $hash{$key}{$value} = 0;
}

foreach my $key (keys %hash) {
    $hash{$key} = [keys %{$hash{$key}}];
}

print Dumper \%hash;

__DATA__
5    25
6    27
5    24
5    23
6    29
6    30
4    22
5    25
6    27
4    22
4    21