First off, for the case of catching STDERR when running another program, check out IPC::Open3, which has been in the standard perl distribution for years.

Right, but this is not apropos to my question. By the way, whenever I think of IPC::Open3, I think instead of IPC::Run which is a much more tractable solution to the same problem set IMO</preaching>

As for your proposed solution. It almost works. I n order for STDERR to be effectively repointed to $module_stderr you have to explicitly close the filehandle and then reopen it. (this is a special requirement that only applies to STDOUT and STDERR when being repointed to an in-memory file). After adding this it does appear that STDERR is in fact repointed somewhere, but it's not clear to me where. It certainly isn't going into that in-memory file variable ($module_stderr). However, if you repoint STDERR to a real file rather than one of these funky in-memory file scalars, then the stderr does in fact end up in that file. Here is an example script which demonstrates the difference

my $module_stderr = '';
close STDERR;
open(STDERR, '>', \$module_stderr);
open F, "perl -e \"warn 'hi'\" |" or die "$!";
while( <F>) {
   print 'out -> '.$_."\n";
}
close F;

print "err -> ". $module_stderr;
print "\n";
open(STDERR, '>', 'testfile') or die "$!";
open F, "perl -e \"warn 'hi'\" |" or die "$!";
while( <F>) {
   print 'out -> '.$_."\n";
}
close F;
close STDERR;
print "err -> ". `cat testfile`;
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The output I get on my setup (Windows XP, ActivePerl 5.8.7) is:

err ->
err -> hi at -e line 1.
[download]

I'm not sure where the standard err out is going in the first case; it's not in the variable and it doesn't show up in the normal terminal output either. Maybe it's just going to /dev/null or somewhere else in the ether. Now if I could just figure out a way to get this to work without using a temporary file, I'd be a happy camper...