USAGEMSG is a constant. A simpler example would be: 
use constant MYCONST => 'hi';

print MYCONST;

__END__

output:
hi

[download]
The '<<USAGE' part is a here document. It's basically a way of defining your own quotes, so everything between the '<<USAGE;' and the final 'USAGE' is a quoted string.

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

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