#!/usr/bin/perl -w
use strict;
use CGI;

# create CGI object
my $q = new CGI;

# create list of fields to check
my @non_empty_form_fields = qw(name email address tel);

# store errors here
my @err = ();

# check fields exist
for (@non_empty_form_fields) {
  unless ( $q->param($_) ) {
    push @err, "You must complete the '$_' field";
  }
}

# if error, represent
if (@err) {
  # read in form
  open (FORM,'/path/to/form.html') || die $!;
  my $form = join '', (<FORM>);
  close(FORM);

  # create error message
  my $err_message = $q->h3('Error').
                    $q->p('I could not process the form:').
                    $q->ul(
                        $q->li([@err])
                    ).
                    $q->p('Check your input and resubmit.');

  # put message in page
  $form =~ s/<!--error_message-->/$err_message/is;

  # show page
  print $q->header,
        $form;
  exit(0);
}

# otherwise input ok, so do whatever...