More as a general point about this class of problem, it's my turn to repeat the well-worn advice about "whenever you start finding yourself with variables ending with incremental numbers, it's time to use arrays instead."

There's nothing at all wrong with assigning an array to a grep'ed version of itself, any more than there is map'ing one - you really don't need @cards1, @cards2 etc.

As Zaxo said, shuffling and popping would be better here, but should you wish to do this (or similar) "in place" as you are doing, something like this would be easier...

while (@hand < $num_to_deal) {
   return "no more cards" unless @cards;
   push @hand,$cards[int(rand(@cards))];
   @cards = grep{$_ ne $hand[$#hand]}@cards;
}
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NB - I'm not advocating this code - as others have said, the grep is nasty - just pointing out a principle :)

Cheers, Ben.