Perl syntax is complexer than we think, since simple things can have multiple means. A word, in your case "foo", declared as it was, can be:

  foo = sub foo {}
  foo = string "foo"
  foo = IO handler *foo
  foo = constant, that actually is: sub foo { "constant" }
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So, when the Perl syntax is parsed, first it will check if we have a sub with that name, if not, it will treat it as a string. As you can see, in your code, sub foo will return the string "foo".

Next time, when calling a function, always use () or &:

  foo()
  # or
  &foo # that is the same of foo(@_)
  # or just to be explicit:
  &foo()
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Take a look in perlsub... And http://perldoc.com is your friend.

Now about the messages of the "use strict" & "use warning" corporation, forget, since know that we need to use that doesn't means that we are goo programmers. There's a lot of things that we can't do with all the strict options enabled, and with all the warnings enabled. And Perl is here to let the developer to be free.

But use at least "use strict qw(vars)" will save you a lot of time.