If whitespace does not truly matter, how does the interpreter know when a line ends?

Because its not absolutely true. 'perldoc perlsyn'

Perl is a free-form language, you can format and indent it however you
like. Whitespace mostly serves to separate tokens, unlike languages like
Python where it is an important part of the syntax.

...

Text from a "#" character until the end of the line is a comment, and is
ignored. Exceptions include "#" inside a string or regular expression.

update: perldoc perlintro'

Comments start with a hash symbol and run to the end of the line

        # This is a comment
[download]

Whitespace is irrelevant:

        print 
            "Hello, world"
            ;
[download]

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