Yeah, but that's how it is called in the docs.

And you will see that it behaves like an instance of IO::File - like other filehandles do.

  DB<100> _->test
Can't locate object method "test" via package "IO::File" at (eval 16)[
+multi_perl5db.pl:644] line 2.
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And this was the point which made me stumble when I tried to declare a package _; some weeks ago.