$_ is not used in the loop. @_ and $#_ (last index of @_) actually refer to the parameters passed into the sub.

Here's a more efficient shuffling algorithm, especially on large arrays:

use strict;
use warnings;
use Data::Dumper;

my @cards = qw/ A 2 3 4 5 6 7 8 9 10 J Q K /;

@cards = shuffle_Deck(@cards);
print Dumper(\@cards);

sub shuffle_Deck {
    my @new = ();

    for (@_) {
        my $i = int rand($#new+1);
        push @new, $new[$i];
        $new[$i] = $_;
    };

    return(@new);
}
[download]