use strict;

my %hash = ();
my ($first, $second);
while (<DATA>)
{
    ($first, $second) = split;
    push (@{$hash{$first}}, $second); # from Perl Cookbook 5.7
}

while ((my $key, my $value) = each %hash) {
    print "the Key is: $key\n";
    print join("\n",@$value),"\n\n";
}

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