Filling a string with Text::Template

[Anonymous Monk]

Hello,
I'm not able to fill a string as in the example below with Text::Template. Where is the mistake?
Thanks

use strict;
use Text::Template;

my $user_info = 'u1';
my $site = 'a';
my $string = './{$user_info}.{$site}.txt';
my $template = Text::Template->new(TYPE =>'STRING',SOURCE=>$string);
$string = $template->fill_in({site=>$site,user_info=>$user_info});
print "$string\n";