Without telling us what the script should do it is very difficult telling you exactly whats wrong. So I can only guess what you want

1) In your subroutine shuffle you use the global variables @starting_deck and @random_card, this probably is wrong when you call shuffle with $file as parameter. It also makes your script behave quite random, more about this later. Generally you should avoid using global variables in subroutines and give all necessary information as a parameter to the subroutine.

2) You use "my $card= shift(@_);". This is ok if you only call the subroutine with one card, but when you do "shuffle(@starting_deck)", you only get to use the first value. What you probably meant was something like this:

my @newdeck= shuffle(@starting_deck);

sub shuffle {
  my @cards= @_;
  foreach (1...30) {
    my @cut= pop(@cards), shift (@cards), pop(@cards) ...
    ...
    push @cards, @cut;
  }
  ...
  return @˘ards;
}
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3) You try to for example substitue 'C' with Clubs'. If you do that more than once, you will produce 'Clubslubs'. So just do that once, maybe in a different subroutine (because really it has nothing to do with shuffling), before or after the shuffling.

Debugging: If your program is not working like you expect, put in 'print' statements to tell you what the program does. Even better, use Data::Dumper to print out your variables and you will get insights into its inner workings

For example you could have added to your old shuffle subroutine something like

use Data::Dumper; # <-better put this at the beginning of your script,
+ but works here too

print Dumper(\@starting_deck);
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You would have noticed then that your subroutine is only executed twice, because you change @random_card in the subroutine while your main script is stepping through @random_card in the foreach loop. @random_card gets emptied and so the foreach ends prematurely. If this is on purpose, ok, but don't think for a moment you can ever return to this script in a few months and change anything without completely breaking the script.