$ cat input.txt
Put the $var1 in the $var2, please.
$ cat filltemplate.pl
use Template;
use strict;
use warnings;

my $vars = {
   var1 => 'cookies',
   var2 => 'oven',
};

my $tt = Template->new({INTERPOLATE => 1});
$tt->process('input.txt', $vars) || die $tt->error();
$ perl filltemplate.pl
Put the cookies in the oven, please.
[download]

Template does much more, of course.