sub card {

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

# these aren't used, so they should be removed
my $guess;
my $i = 0;

# there is no need for these vars to be global:
our $num;
our $negative = "negative";

# declare them as lexicals instead
my $num;
my $negative = "negative";

# declaring $num as lexical you don't need to localize:
local($num) = @_;

# this works here, but do you really understand what its doing?
# hint: list context
my ($num) = @_;

# this is better
my $num = $_[0]:

# or this:
my $num = shift @_;

# commonly shortened (@_ is implicit) to:
my $num = shift;

# for the rest, notice how much simpler the logic
# is in moritz reply above

# so putting it all together:
sub card{

    my $num = shift;
    my $negative = "negative";
    my @card_map = qw(zero one two three four five six seven eight nine);
    my $return_value;

    if ($num < 0){
        $num = -$num;
        $return_value = $negative;
    }

    $return_value .= "$card_map[$num]";
    return $return_value;
}

# and simplified further:
sub card{

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

    if ($num < 0){
        $num = -$num;
        return "negative $card_map[$num]";
    }
    return $card_map[$num];
}