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`;

##</code><code>##

err ->
err -> hi at -e line 1.