How about something like this?

unless ($form{'searchstring'}) {
    $form{'searchstring'}= '[a-zA-Z]';
}
if ($desc =~ /$form{'searchstring'}/i) {
...
}
[download]

It's unclear from your code, but I assume you know that this has to be done in the post-processing of the form, and not at the same time you are asking the form....

Added:
You could always do something like this when you are asking the form: <input type="text" name="searchstring" size="18" VALUE="any character"> Of course, "any character" will appear in the search box every time, and the program will default to searching for "any character" unless you add something like this:

 
if ($form{'searchstring'} eq "any character") {
    $form{'searchstring'}= '[a-zA-Z]';
}
[download]