Update 2: I hacked myself a workaround by removing the overloaded string comparison of DateTime. This is horribly ugly and there must be a better way :-(

Yes, that way is to not put method calls in templates. Is it really so hard to use the following?

my $input = \'[% calendar %]';
$template->process($input, 
  { calendar => DateTime->now->ymd(".") } );
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I realize this is just a matter of style, but I really can't think of any good reason for such a simple operation not to occur in your program. Your program should assemble data, and your template should display it.

(Caveat: For efficiency reasons, sometimes it's better to pass an iterator to a template instead of a fully constructed data structure. But this sort of thing should be the exception, not the rule, IMHO.)