THe example above was mod_perl, you probably got that error because 1) you are not using mod_perl or 2) you are calling your Apache object something other than $r. In the case of #1, a CGI.pm example follows:

use CGI qw(:standard);
print header(
    'type'       => 'application/pdf',
    'attachment' => 'adobe.pdf'
);
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Edit by castaway: Closed small tag in signature