Mike

Many thanks for your reply and sorry for taking a while to respond...

<confusion>
Er, yes, that does work. Ok, then. Given that open(STDERR, '|-'); was actually the first thing I tried and it did not work for me, I obviously did something stupid. It turns out that I did two stupid things. The first was that I did not try all the alternatives against a simple test case. The second was that I did not read the source of P::RD carefully enough.

...Time passes while Kevin reads P::RD properly this time...

The output I am concerned about is actually written to a format, not directly to STDERR. Why does this matter? Because the format is opened by dup(2)ing STDERR, which gives a fresh file descriptor. So, I can sit there in the child waiting for output on STDERR until I am blue in the face and it will not get me anywhere.
</confusion>

So, what now? It does not seem as though I can open an output filter, since I do not know to which file descriptor I should listen. Perhaps I could spin around all of them, but that sounds pretty obscure.

How about tie'ing the format to a suitable class? Well, this partially works. As it happens, P::RD both prints and writes to the format. The prints come through fine but the writes do not. It seems as though writes to a format cannot be tied. See the example code below:

use strict;
use warnings;

package Log::OPG::Info;
use Tie::Handle;
use base qw(Tie::StdHandle);
sub WRITE { print "Good\n"; }


package main;

format STDERR =
Bad
.

tie *STDERR, 'Log::OPG::Info';
print STDERR '';
write STDERR;
[download]

Whereas I hope for the output

Good
Good
[download]

I actually get

Good
Bad
[download]

So, either I am missing something obvious, or the follow-up question is:

"How does one tie writes to a format?"

Cheers
Kevin

P.S. I faintly remember that it is possible to name individual slots in hash table entries, along the lines of $::slot{SCALAR} and $::slot{FORMAT}. Perhaps that might form the basis for a solution, but the details escape me.