sub f {
   my ($arg1) = @_;
   print(defined($arg1) ? $arg1 : '[undef]', "\n");
}

sub t1 { print("t1: "); f(); }
sub t2 { print("t2: "); &f;  }

t1('moo');  # t1: [undef]
t2('moo');  # t2: moo