How did "Cow" become an argument when I didn't specify it anywhere?

That's what The Arrow Operator does. From "Method Invocation" in perlobj:

When you call a method, the thing on the left side of the arrow is passed as the first argument to the method. That means when we call Critter->new(), the new() method receives the string "Critter" as its first argument. When we call $fred->speak(), the $fred variable is passed as the first argument to speak().

In reply to Re: Trying to understand method calling in OOP by haukex
in thread [SOLVED]: Trying to understand method calling in OOP by Perl300

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