This is a good opportunity to learn unit testing with Test::More et.al. :

# 718965.pm

use strict;
use warnings;

my @card_map = qw(zero one two three four five six seven eight nine);

sub card {
  my $num = shift;
  my $negative = "negative";

  if ($card_map[$num]) {
    return $card_map[$num];
  } else {
    return $num;
  }

  if ($num > 0) {
    return $num = -$num;
  }   

  if ($card_map[$num]) {
    return $negative . $card_map[$num];  
  } else {
    return $negative . $num;              
  }   
}   

use Test::More;
plan tests => 3;

is(card(5), 'five', 'Expect card(5) to return "five"');
is(card(6), 'six', 'Expect card(6) to return "six"');
is(card(-6), 'negative six', 'Expect card(-6) to return "negative six"
+');

__END__

$ prove 718965.pm
718965....NOK 3
#   Failed test 'Expect card(-6) to return "negative six"'
#   at 718965.pm line 32.
#          got: 'four'
#     expected: 'negative six'
# Looks like you failed 1 test of 3.
718965....dubious
        Test returned status 1 (wstat 256, 0x100)
DIED. FAILED test 3
        Failed 1/3 tests, 66.67% okay
Failed Test Stat Wstat Total Fail  Failed  List of Failed
----------------------------------------------------------------------
+---------
718965.pm      1   256     3    1  33.33%  3
Failed 1/1 test scripts, 0.00% okay. 1/3 subtests failed, 66.67% okay.
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