Quoth the Camel (3ed, p927):

You used the syntax of a method call, but the slot filled by the objec
+t reference or package name contains an expression that returns a def
+ined value that is neither an object reference nor a package name. So
+mething like this will reproduce the error:

$BADREF = 42;
process $BADREF 1,2,3;
$BADREF->process(1,2,3);
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To make a long story short, you probably have a package named Voice, with a method named Calendar(). Instead of a class method invocation, you're calling &Voice::Calendar(), which returns something that is a true value, but not an object reference or package. Example:

$ perl -e 'package Foo::Bar; sub Foo {1}; sub Foo::Bar {1}; package ma
+in; Foo::Bar->Foo'
Can't call method "Foo" without a package or object reference at -e li
+ne 1.
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To prevent this confusion, you can quote the package name:

'Foo::Bar'->Foo
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conv

Update: Of course there's a package named Voice, I should read more carefully.

For fun: You could also have made the Calendar() method in the Voice package return 'Voice::Calendar', which Perl would take as the package name, but that would probably break other stuff. (: