I don't think it is

# If the write handle is a dup give it away entirely, close my copy
# of it.
xclose $dad_wtr if $dup_wtr;
[download]

it seems like my script is actually dying at the open3 step

Oops, yeah, I knew that. So it's an unrelated problem.

Let's find out more details:

$ perl -MCarp=verbose a.pl
open3: close(4) failed: Bad file descriptor at /usr/share/perl/5.8/IPC
+/Open3.pm line 70
        IPC::Open3::xclose(4) called at /usr/share/perl/5.8/IPC/Open3.
+pm line 218
        IPC::Open3::_open3('open3', 'main', '<&4', '>&STDOUT', '>&STDE
+RR', '/bin/cat') called at /usr/share/perl/5.8/IPC/Open3.pm line 229
        IPC::Open3::open3('<&4', '>&STDOUT', '>&STDERR', '/bin/cat') c
+alled at a.pl line 21
[download]

Doh! It's calling close("4")!

xclose $dad_wtr if $dup_wtr;
[download]

should be

if ($dup_wtr) {
    if (fh_is_fd($dad_wtr)) {
        xclose $dad_wtr;
    } else {
        xfdclose xfileno($dad_wtr);
    }
}
[download]

Defining xfdclose is left as a task to the reader.