USAGEMSG is the name of the constant being defined.

USAGE is a string of characters being used to mark the end of the Heredoc (a way of defining the value of a string) block. You can read about the syntax in perldoc perlop. Grep for "<<EOF".

In reply to Re: Meaning of 'use constant USAGEMSG = > ...' by dorward
in thread Meaning of 'use constant USAGEMSG = > ...' by rjsaulakh

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