From the manual page of fork

               Beginning with v5.6.0, Perl will attempt to flush
               all files opened for output before forking the
               child process, but this may not be supported on
               some platforms (see the perlport manpage).  To be
               safe, you may need to set "$|" ($AUTOFLUSH in
               English) or call the "autoflush()" method of
               "IO::Handle" on any open handles in order to avoid
               duplicate output.

Abigail