#!/usr/bin/perl

use strict;
use warnings;
use feature qw{ state };

sub html {
    my ($text) = @_;

    # Make the header idempotent
    state $b_header++ or print "Content-type: text/html\n\n";

    print $text;
}

html("<br>This is some html, prior to which a header is printed\n");
html("<br>This is too (but the header was printed already)\n");

##</code><code>##

Content-type: text/html

<br>This is some html, prior to which a header is printed
<br>This is too (but the header was printed already)