Use this descendent of CGI

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;
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If you use this class instead of CGI, and call header in the object oriented sense, it should accomplish what you want.

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