I've put some comments in the code. You didn't say which parts of it were giving you trouble, so don't get upset if I tell you something you already know. If you have more detailed questions about any of this, or want to correct my interpretation, please feel free. :)

For more help on Perl functions such as fork, wait, etc, do a 'perldoc perlfunc' on your system along with Super Search of this site.

#!/usr/local/bin/perl -w
# I assume @dates contains a list of formatted date values
# which correspond to a method of naming directories used in the
# rest of the code.
foreach $datdir (@dates)
{
   # full or 'absolute' file paths based on @dates value
   $fullpath = (join ('',$ypath, $datdir));
   $fulltmp = (join ('', $ytmp, $datdir));

   # Ask the OS to make a child process and save its process id in $pi
+d
   my $pid = fork();
   if ($pid == 0)

   #-- start child process code
   {
      # Do 'ls -ld $fullpath' on remote system $yte and put its output
+ in $what
      $what = `rsh $yte ls -ld $fullpath`;
      # If that output contains any non-whitespace characters
      # (which AFAICS looks like it would always be true)
      if ($what =~/\S/)
      {
         # Hit the remote system $yte again and make $fulltmp
         $what2 = `rsh $yte mkdir -p $fulltmp`;
         # If the output of the mkdir does not contain any non-whitesp
+ace
         # (the non-whitespace would be an error msg of some sort)
         if ($what2 !~/\S/)
         {
            # This is probably a 'success'
            exit 1;
         }
      }
      # This is probably 'fail'
      exit 0;
   }
   #-- end child process code

   # Store $fullpath in %paths hash along with the pid of the child pr
+ocess
   $paths{$pid} = $fullpath;

   # NB The success (1) or failure (0) of the child process code
   # revolves around whether the *$fulltmp* directory can be created
   # though it is $fullpath which is stored

   # increment child job count
   $njobs++;
   # Child jobs "brake" - wait for a child job to complete if
   # there are more than $maxnjobs children currently running
   # before continuing the foreach loop
   while ($njobs >= $maxnjobs)
   {
      $waitpid = wait();
      # Check return value of child process
      if ($?)
      {
         # Collect 'valid' $fullpath values in @vdates
         push(@vdates, $paths{$waitpid});
      }
      $njobs--;
   }
}
#  Collect the exit status and save any 'valid' $fullpath values from
#  the rest of the child jobs
while ($njobs > 0)
{
   $waitpid = wait();
   if ($?)
   {
      push(@vdates, $paths{$waitpid});
   }
   $njobs--;
}
# Sort 'valid $fullpaths' lexically
my @sorteddates = sort(@vdates);
# Return that sorted list
return(@sorteddates);
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