This only tells me what variables have been passed to the template, but not what variables the template actually uses:

#!perl -w
use strict;

use Template;
use Template::Plugin::Stash;

my $t = Template->new();

my $tmpl = <<'TMPL';
Name:     [% user_name %]
Password: [% user_password %]

---
[% USE Stash %]
[% USE Dumper %]
[% Dumper.dump( Stash.stash() ) %]
---
TMPL

$t->process( \$tmpl, { user_name => 'Corion', user_password => 'secret
+', user_email => 'corion@corion.net'}, \my $output );
print $output;
[download]

gives

C:\Projekte>perl -w tmp.pl
Name:     Corion
Password: secret

---


[% Dumper.dump( Stash.stash() )
---

C:\Projekte>perl -w tmp.pl
Name:     Corion
Password: secret

---


$VAR1 = {
          'global' => {},
          'user_name' => 'Corion',
          'user_password' => 'secret',
          'user_email' => 'corion@corion.net'
        };

---
[download]

and user_email is listed there, even though it is not used in the template anywhere.