Preview != create :(

Capture::Tiny is indeed the way to go, with a small gotcha:

$ cat test.pl
use 5.18.2;
use warnings;

use Capture::Tiny "capture";

my ($out, $err, $ext) = capture {
    # Local scope
    print STDERR "Output to STDERR\n";
    warn "Warn!\n";
    system "ls /tmp/bjfgerfbv";
    qx{ls /tmp/jhgbejgrbe};
    };

# Previous scope
print STDERR "Hello world\n";
system "ls /tmp/foobar2";

print STDERR "OUT: $_\n" for split m/\n/ => $out;
print STDERR "ERR: $_\n" for split m/\n/ => $err;
$ perl test.pl
Hello world
/bin/ls: cannot access /tmp/foobar2: No such file or directory
ERR: Output to STDERR
ERR: Warn!
ERR: /bin/ls: cannot access /tmp/bjfgerfbv: No such file or directory
ERR: /bin/ls: cannot access /tmp/jhgbejgrbe: No such file or directory
[download]

The gotcha is that STDERR inside capture is not unbuffered, so if your last print STDERR "foo"; does not have a trailing newline, it is lost. That might change in a newer release (at least I hope)