I'm a little confused. You talk about using __END__ but then you mention passing the DATA filehandle. I assume you meant to use __DATA__ instead.

HTML::Template's new() method can also take text as the template, it doesn't have to be a filehandle. Per the docs:

Alternately you can use:

  my $t = HTML::Template->new( scalarref => $ref_to_template_text, 
                               option => 'value' 
                             );
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and

  my $t = HTML::Template->new( arrayref => $ref_to_array_of_lines , 
                               option => 'value' 
                             );
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These initialize the template from in-memory resources.

It looks like you can just slurp in your template (using while( my $line = __DATA__ ) or similar) and then pass the string to HTML::Template->new().