That's because @newdeck is a global variable, and you don't clear it.

I recommend to use strict; use warnings; and declare my @newdeck inside the shuffle sub.

There's also a shuffle function in the core module List::Util, which is tested and works.

Thank you moritz! Declaring the return value as private fixed my issue!

Because you don't clear the global variable @newdeck before calling shuffle() again.

Use the strict and warnings, Luke!

Your code doesn't run at all in fact. @array does not exist and your prototyped sub is declared too late for the prototype to be checked.

As already suggested use strictures (use strict; use warnings; - see The strictures, according to Seuss).

Don't use prototyped subs. In your sample code the prototype says "take no parameters" but @array is almost certainly intended to be a parameter (you surely aren't using global variables are you?).

A first cut at cleaning up the code could look like:

#!/usr/bin/perl
use strict;
use warnings;

my @deck = qw(card1 card2 card3 card4);

my @shuffle1 = shuffle (@deck);
my @shuffle2 = shuffle (@shuffle1);

print "@shuffle1\n";
print "@shuffle2\n";

sub shuffle {
    my $first = 0;
    my $last = $#_;
    my @newdeck;

    while ($first < $last) {
        push @newdeck, $_[$last--], $_[$first++];
    }

    return @newdeck;
}
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Using a somewhat more Perlish (and probably harder to understand) approach to implementing shuffle may get you to:

sub shuffle {
    my @newdeck;

    push @newdeck, @_[-$_ - 1, $_] for 0 .. @_ / 2 - 1;
    return @newdeck;
}
[download]

Both versions print:

card4 card1 card3 card2
card2 card4 card3 card1
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  push @newdeck, pop, shift  while @_;
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but every version fails in case of odd number of cards ...

  push @newdeck, grep defined, pop, shift  while @_;
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Next assignment:

Write a function that creates a deck of cards. The strings representing each card should look like "A H", which stands for "Ace of hearts". The function should return an array containing 52 strings: one for each card in a standard deck of cards.