Just do everything in cents. The following or some variation should work fine:

use strict;
use warnings;

my $payment_total = 415;
my $order_total = 420;
my $difference = $order_total - $payment_total;
print cash($difference);

sub cash {
    $_[0] = int($_[0] + .5);
    $_[0] /= 100 if $_[0] != 0;
    return sprintf('$%.2f', $_[0])
}
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