Most likely you can do this with the World's Simplest Templating Engine:

use File::Slurp 'read_file';
my $template = read_file($template_name);
1 
  while $template =~ s/\[%\s*INCLUDE\s(.*?)\s*%\]/read_file($1)/msge;
print $template;
[download]

This will not handle include parameters and will not search different directories, but you can add that by replacing read_file by a subroutine more suited to your needs.

Update: Corrected regular expression according to pc88mxer's comment