blackjudas,
The following code works fine for me - just as you want it to:

#!/usr/bin/perl -w
use strict;
my %columns = (
    1 => 1,
    2 => 1,
    3 => 1,
    4 => 1
);

my @discounts = qw( 1 2 1 3 1 4 1 5);

for my $discount (@discounts) {
    if ($discount == 1) {
        my $key_num = scalar keys %columns;
        $key_num++;
        $columns{$key_num} = 1;
    }
}
print "$_ : $columns{$_}\n" foreach (keys %columns);
[download]

Cheers - L~R

Update: As others have pointed out - if %columns is already populated and the keys are not sequential starting from one - you run the risk of this not working. This could also be a problem if you delete keys elsewhere. For instance:

17 => 1
3  => 1
[download]

There are 2 keys, add one you get 3, but guess what - 3 already exists. You may want to check to see if the key exists. I think there is a better way to accomplish whatever you are trying to do.