#!/usr/bin/perl

# save original STDERR
open my $saved_stderr, ">&STDERR";

# create a pipe, which we'll use to read STDERR
local(*RH, *WH);
pipe RH, WH;

# connect the writing side of the pipe to STDERR, with
# STDERR being (and remaining) fileno 2 (!)
open STDERR, ">&WH" or die "open: $!";

# debug: verify that fileno(STDERR) really is 2
printf "fileno(STDERR): %d\n", fileno(STDERR);

# execute external command
my $out = `perl -e "print 'hello world'; die('this is a fatal error')"`;
my $ret = $?;

# close WH to avoid buffering issues (pipes are buffered)
close WH;
# read output (one line)
# (todo: fix so it doesn't block when there's nothing to read!)
my $err = <RH>;
close RH;

# restore original STDERR
open STDERR, ">&", $saved_stderr or die "open: $!";

print "return value:    $ret\n";
print "captured stdout: $out\n";
print "captured stderr: $err\n";