According to the docs, the desired syntax for your application is

$template->output(print_to => \*FILE);
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In general, the syntax for named parameters is

func(name => $value, ...);
$obj->method(name => $value, ...);
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optionally with a hyphen in front of the name, which makes it look like a command line option (and which should ring the "familiarity" bell); and which makes it stand out a little more:

func(-name => $value, ...);
$obj->method(-name => $value, ...);
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In case you find this baffling: the autoquoting rule for the left hand side of "=>" also applies to barewords preceded with a hyphen. So, (-name => 'value') is interpreted by Perl as ('-name', 'value').

But, from the source I would gather that HTML::Template doesn't honour this tradition, by which I mean it likely won't recognize $template->output(-print_to => \*FILE); . However, I musty admit I haven't tested it.