The apprentice, after taking aspirin for the pain in his forehead, finally caves after several hours...

Some time ago, a certain monk (privately) advised the following after seeing my poor attempts to verify that a certain CGI parameter was actually a number, e.g. a non-blank, Base-10 numeric value that may (or may not) contain decimal points and/or hyphens masquerading as negative number indicators:

Best perlop check? Try this: eval {local $^W = 1; $num + 0}; # Now check $@

Okay. So I'm trying to suit the action to these words and am not having much success:

#!/usr/bin/perl -wT
use strict;
$|++;

use CGI qw( :standard );

my $result = '';
my %params = param();
if ( my $error = cgi_error() ) 
{
  print header( -status => $error );
  exit 0;
}

my $invalue = param( 'INVALUE' );
if ( defined $invalue ) 
 {
    eval{ local $^W = 1; $invalue + 0 };   ### Is Number?  (Line 19)
    if ( $@ ) 
       { $result = "$invalue is *not* a Number; Details: $@" }
    else
       { $result = "$invalue appears to be a number."; }
  }
else
  { $result = "Enter a value to test." }

print header( "text/html" ), 
      start_html('Test'),
      h1('Validation'),
      start_form(),
      p( $result ),
      p( "Please enter a number to test: ", textfield( 'INVALUE', '' )
+ ),
      p, submit,
      end_form, hr,"\n";
print end_html;

exit 1;
[download]

As the more experienced of you will recognize, this is resulting in values such as "20" as being recognized as a number. Sadly, so are "Shoe", "Fred" and other patently non-numeric values, by which (just to be perfectly clear to the most rententive) are *not* numeric in the locale, idiom, and operational parameters that I'm trying to communicate in.

My petitions are:

• Have I implemented the mysterious monk's advice properly? (Probably not; gentle correction encouraged).

• Why are "Shoe" and "Fred" being recognized as numbers? Try it out; you'll see.

• How do I get rid of the "Useless use of addition in void context at scriptname.cgi line 19." error that's appearing when I run this from the command-line? Perhaps more appropriately, how do I do so and achieve my primary goal, e.g. confirming that the user of my CGI form has indeed entered a non-blank number (including "0")?

Yes, I am aware of the Cookbook and its examples; you'll note this monk had a different approach in mind. I'm trying to get it to work. A gentle nudge would be greatly appreciated.

Many thanks in advance...

--f