To check if the value is 0, how about :   if ($CGI->param('fieldname') == 0)...

Keep in mind that "three" == 0. To check if the entered value equals the single character 0, use a string comparison: $something eq '0'.
It's generally a good idea to warn if the user didn't enter nothing or digits. Or, using reversed logic: warn if a non-digit character is found:

die "\$foo contains a non-digit character!\n" if $foo =~ /\D/;
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++ vs lbh qrpbqrq guvf hfvat n ge va Crey :)
Nabgure bar vs lbh qvq fb jvgubhg ernqvat n znahny svefg.
-- vs lbh hfrq OFQ pnrfne ;)
    - Whreq