If the template is very small, probably the most efficient method is to print directly:

print <<OUT;
First line!
This line has a $var in it.
Second line...
Thanks for reading this!
OUT
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If your template is large but has only a few insertion points, you can read the file into a string and use index or rindex:

use strict;
use warnings;

my ($handle, $text, $insert);

open($handle, 'template.txt');
read($handle, $text, 10000);
close($handle);

$insert = index($text, '<!-- INSERT -->');

print substr($text, 0, $insert);
print 'My inserted data goes here!';
print substr($text, $insert);
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Beyond that, a formal templating system or module is probably best, or you can always use PHP, which is designed for embedding in pages:

Blah blah blah
I'm inserting a variable here: <?php echo $var; ?>
And again here: <?php echo $another; ?>
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I still like using PHP for simple things.