From perldoc -f 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 perlport). 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.

Forking cannot easily be replaced with an exec of itself. The process may have done extensive computation for instance. Furthermore, after an exec(), you still have open file handles: STDIN, STDOUT and STDERR.