my $dir = "/path-to/some-dir";
while () {
   opendir(DIR, $dir) || do {
      # my error code here...
   };
   for my $filefound (grep /\.foo$/, readdir DIR) {
      my $fullPath = File::Spec->catdir($dir, $filefound);
      # immediately rename the file
      my $newPath;
      ($newPath = $fullPath) =~ s/foo/bar/;
      rename ($fullPath, $newPath) || do {
         # my error code here...
      };
      my $kidPid;
      unless (defined $kidPid = fork()) {
         # my error code here...
      }
      next if $kidPid; # parent continues with dir grep
      # I'm the child, I process the file then exit
   }
}