Good first attempt!

Here's my version:

sub card {
  my $num = shift;
  my $result;
  my @card_map = qw(zero one two three four five six seven eight nine)
+;
  
  if ($num < 0) { 
    $result .= "negative ";
    $num = -$num;
  }
  
  if ($card_map[$num]) {
    $result .= $card_map[$num];
  } else {
    $result .= $num;
  }
  
  return $result; 
}
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