You need to actually declare function test_func, which in Perl you do with the sub keyword:

sub test_func {
   print "hello\n";
}
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See perlfunc and perlsyn for details.

Oh, and please don't use Switch;, it can go horribly wrong, and is deprecated in favor of given/when, which are available starting from perl 5.10.0 (current old-old release).