Error checking in perl

I have got an error check that checks whether a user has entered any data into a field the code is as follows:

if ($FORM_DATA{firstname} eq "") {
      print"Please enter your firstname<BR>";
}

The above code works fine.  I am however wondering if anyone knows how to check if the user has entered digits or alphabetic characters.

I would like an  Incorrect message to display as above if the user types digits into a field where alphabetical characters ahould be and vice versa does anyone have any ideas.

For example a credit card number field should only contain digits and no alphabetical characters, if the user does enter any alphabetical characters then an Incorrect message is displayed.

I have looked in books but can only find error checking for a blank field.

Any help greatly appreciated.

Anne. email:  anne.smith@lineone.net