Whatever method you use to create the checkboxes should call the same method to validate the checkboxes to check their input. The first

my $checkbox_fields = get_checkbox_fields();

for my $field (@{$checkbox_fields}) {
  $in{$field} = $query->param($field) ? 'checked' : 'not checked';
}

sub get_checkbox_fields {
  return [qw(c_test1 c_test2 c_test3)]
}
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If you use the get_checkbox_fields() to generate the checkboxes on your web page, then you can use it to verify the data you get from the web page.

Looping through the fields using a regexp can lead to errors with conflicting field names or someone inserting extra fields into the submission. You can never rely on a naming convention to classify data you receive from a web page. Always know exactly what you expect and you do not have to worry about these sorts of issues.