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This is most powerful method with fork creating two identical copies of program -- parent and child
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unless (fork) { ;} # child indicated by fork=0
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; # otherwise fork=child process number for parent
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The child program typically invokes exec which replaces child original by the argument of exec. Meanwhile parent should wait until this exec is complete and child has gone away.
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unless (fork) {
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exec("date"); # child process becomes date command sharing environment with parent
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}
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wait; # parent process waits until date is complete
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The child process need not terminate naturally as with exec() and if child code was for instance
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print FILEHANDLE @hugefile; # in parallel with parent
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exit; # is required else child will continue with parents code whereas we wanted parent and child to work in parallel on separate jobs
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