scalar( @reqform    ) 
    != 
    scalar( $q->param() )

##</code><code>##

    join( $; , sort( @reqform    ) ) 
    ne 
    join( $; , sort( $q->param() ) ) 

##</code><code>##

	while( ( $key, $value) = each %form )
	{
	    push @missing_params, $key if $value eq '';
	}

	if( scalar @missing_params ) # ...

##</code><code>##

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

use CGI;


my $q = new CGI;
my @reqform = qw( var1, var2, var3 );

print $q->header();

if( 
    ( # different number of elements
        scalar( @reqform    ) 
        != 
        scalar( $q->param() )
    )
    || 
    ( # element names do not match
        join( $; , sort( @reqform    ) ) 
        ne 
        join( $; , sort( $q->param() ) ) 
    )
)
{ # print error message for mismatched parameters
    print
        $q->start_html(),
        $q->p( "ERROR: mismatched parameters:" ), 
        $q->p( "required: ", join ' ', sort @reqform    ), 
        $q->p( "passed: ",   join ' ', sort $q->param() ), 
        $q->end_html();
}

# get variables from submitted form
my %form = $q->Vars(); # note: this does not work with isindex tags

my ( $key, $value, @missing_params );

while( ( $key, $value) = each %form )
{
    push @missing_params, $key if $value eq '';
}

if( scalar @missing_params )
{ print error message
    print
        $q->start_html(),
        $q->p( "ERROR: undefined parameter(s): @missing_params" ), 
        $q->end_html();
}

# carry on...