Thank you for your response. I'll do my best to add more detail (without being too detailed).
So at script startup-time, the parent immediately forks the child: 
$socket = socketpair($child_fh, $parent_fh, .....);
$child_fh  -> autoflush();
$parent_fh -> autoflush();
$pid = fork();
....

[download]

So the child and parent can use these file-handles to send and receive.
In the child process: 
while(1)
   { my %INFO = ();
     .... stuff-which-populates-%INFO ...
     my $dumped = Dumper(\%INFO);
     print $parent_fh ":MSG:${dumped}:DONE:\n";
     sleep 10;
   }

[download]
%INFO will always be populated ... it will never be null.
I am mindful that $dumped could be upwards of 10Kbytes.
Hence, I frame $dumped with :MSG: and :DONE:.

The parent process is a Perl/Tk GUI and so uses fileevent() on $child_fh to invoke a callback whenever there is activity on the file-handle.
So the call-back is of the form: 
state $collected;
$num = read($child_fh, $buf, 1e6);
if ($num)
   { $collected .= $buf;
     if ($collected  =~ /:MSG:(.*):DONE:/s)
        { $dumper = $1;
          %info = %{ eval $dumper }
          $collected = "";
        }
   }

[download]
I make sure that the opening and closing framing text (:MSG: and :DONE:) is present, just in case the data is transacted in dribs-and-drabs.
As I write this, I guess it is possible that the asynchronous nature of data being collected in $collected could be of the form:
:MSG:<Dumped-data-from-%INFO>:DONE::MSG:<Dumped-data-from-another-%INFO>:DONE:
And therefore the pattern match will match: <Dumped-data-from-%INFO>:DONE::MSG:<Dumped-data-from-another-%INFO>
which is not in the correct format for the eval.
At the time, I discounted this because of the autoflush ... but perhaps that is a false assumption.

Following on from some of the tips provided, I have inserted: local $SIG{__WARN__} = \&WhatsWrong; and I have defined a function WhatsWrong which can then print out the value of $dumper.
It will take a few weeks for the problem to occur, so I will keep you updated when I have more information.
Thank you very much.

In reply to Re^2: Help requested to find the cause of an "uninitialized value" message by andyok
in thread Help requested to find the cause of an "uninitialized value" message by andyok

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