If you mean the report_handler bareword by "file handle", that's because in the past (before version 5.6) Perl didn't have ability to work with lexical filehandles, and bareword filehandles were used instead. Read Typeglobs and Filehandles and https://www.socialtext.net/perl5/bareword_uppercase_filehandles for more information on bareword filehandles. They are also easier to understand by Perl beginners.