Perl5 リファレンス

       fork    Does a fork(2) system call to create a new process
               running the same program at the same point.  It
               returns the child pid to the parent process, `0'
               to the child process, or `undef' if the fork is
               unsuccessful.  File descriptors (and sometimes
               locks on those descriptors) are shared, while
               everything else is copied.  On most systems
               supporting fork(), great care has gone into making
               it extremely efficient (for example, using copy-
               on-write technology on data pages), making it the
               dominant paradigm for multitasking over the last
               few decades.

               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.

               If you `fork' without ever waiting on your
               children, you will accumulate zombies.  On some
               systems, you can avoid this by setting
               `$SIG{CHLD}' to `"IGNORE"'.  See also the perlipc
               manpage for more examples of forking and reaping
               moribund children.

               Note that if your forked child inherits system
               file descriptors like STDIN and STDOUT that are
               actually connected by a pipe or socket, even if
               you exit, then the remote server (such as, say, a
               CGI script or a backgrounded job launched from a
               remote shell) won't think you're done.  You should
               reopen those to /dev/null if it's any issue.