First, I should qualify all the following with the comment that I use Getopt::Long in all my own work; I've found it sufficiently powerful for all my needs, straight forward to use, and it's CORE so I don't have to worry about library management.

You've got a number of code problems, most of which which would be caught by strict. The strict pragma implements a number of restrictions on your code, such as requiring all variables be either declared with a my or our or have a fully qualified package name. See Use strict warnings and diagnostics or die.

So bugs in the provided code are:

1. On line 3, you call the class method new_parser, but have not wrapped the hash ref you are using as your parameter set in the necessary parentheses, nor have you terminated the line in a semi-colon. This syntax error causes compilation to fail, which tells me you did not try running your code example before posting. See How do I post a question effectively?. The correct call would be:

   $ap = Getopt::ArgParse->new_parser(
     {
         prog => "test_argparse.pl",
         description => "Test the Getopt::Argparse module"
     }
   );
   [download]

   or something close.

2. On lines 9 and 11, you failed to wrap the Scalar and Bool values in quotes, so (since you didn't have strict on) Perl attempted to run subs called Scalar and Bool, got undef and so passed that to the argument constructor. Those lines should be

   $ap->add_argument('-c', '--count', type => 'Scalar');
   
   $ap->add_argument('-f', '--flag', type => 'Bool');
   [download]

   => stringifies the preceding argument, but you must still wrap the trailing argument in quotes. Other than that, it's just a comma.

3. The add_argument method uses the first passed flag name as the name for the field internally. Since you seem to want the field to be named count, not c, that should be

   $ap->add_argument('--count', '-c', type => 'Scalar');
   
   $ap->add_argument('--flag', '-f', type => 'Bool');
   [download]

4. $ap->parse_args() returns a blessed hash reference, which is Perl's version of an object (<--simplification). However, on line 18, you attempt to access a hash named %args. Again, this is something strict would have caught. The correct way to dig about in a hash ref is to use the The Arrow Operator. In that case, line 18 should have read

   if (! defined($args->{'count'})) {
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   HOWEVER, since $args is not just an ordinary hash ref, but an object, it is good practice to use the provided accessors. This protects you from typos in hash keys. The module provides two methods for accessing field names: either via the $args->get_attr(ARG) method, or via the Moose-like getter $args->ARG So you could get your desire result with either

   if (! defined($args->get_attr('count'))) {
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   or

   if (! defined($args->count)) {
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   assuming you've made the argument order correction above.

So, a corrected version of your code that runs as expected is:

#!/usr/bin/perl -w
use strict;

require Getopt::ArgParse;

my $ap = Getopt::ArgParse->new_parser(
  {
      prog => "test_argparse.pl",
      description => "Test the Getopt::Argparse module"
  });

$ap->add_argument('--count', '-c', type => 'Scalar');

$ap->add_argument('--flag', '-f', type => 'Bool');

my $args = $ap->parse_args();

# Now, I am assuming that $args is a pointer to a hash array
# that contains elements for "count" and "flag"

if (! defined($args->count)) {
   print "count not defined\n";
}
[download]