package Some_CGI;
use CGI;

@ISA = qw(CGI);

use strict;

my $is_header_printed = 0;

sub header {
   my $self = shift;

   return if $is_header_printed;
   $is_header_printed = 1;
   $self->SUPER::header();
}

1;

##</code><code>##

...
   my $form = Some_CGI->new();
...
   print $form->header();
...