This seems to be some intermediate or side effect goal to me. TT's output is a string, so for that purpose undef and the empty string are equivalent. Maybe you have something like [% IF hashref.defined('value1') %]...[% END %] later on, or you want to pass a value back to the Perl code which called TT's process? For these situations you can [% hashref.delete('value1') %] which should do the trick unless you also need to distinguish between keys which exist from keys with an undefined value.