End of script output before headers - Perl

Question

Why so:

print "Content-type: text/html\n\n";
print "<H1>Hello World</H1>\n";

Works!

And so:

my $tpl = HTML::Template->new(filename => 'index.html');
print "Content-Type: text/html\n\n" , $tpl->output

I have the error:

The server encountered an Internal error and was Unable to complete your request.

Error message: End of script output before headers

• I already checked permission
• Already insert simple html

I’m having trouble, at the time I try to import an HTML and I really need to do it. Would someone give me a light?