What you'll need to do is, as described above, create some type of data structure out of the file beforehand. What you might want to do is start with a chunk of code like

my($category, $subcategory) = split(/&/, $ENV{'QUERY_STRING'});

if($subcategory ne "") {
    # Read in the file and look for the subcategory
    # specified, as you are already doing
}
else {
    # Read in the file and count the values in the
    # subcategory, as per turnstep's answer above.
}
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This would get you the desired behavior of having a CGI that can handle with and without a subcategory.