The real problem here is [...] Nothing you do to the current process Perl handles or CRT handles will have any influence upon the created process at all.

Actually, no, it just works:

X:\> type stdout.pl
#!/usr/bin/perl -w
use strict;

require Win32::Process;

open STDOUT, "> stdout.txt"
    or  die "Can't write stdout.txt: $!\n";

my $proc;
Win32::Process::Create( $proc, $ENV{COMSPEC}, "/c dir", 1, 0, "." )
    or  die "Can't run 'dir': $^E\n";

X:\> perl stdout.pl

X:\> type stdout.txt
 Volume in drive X is NSA_SECRETS
 Volume Serial Number is 007A-DEAD

 Directory of X:\

2008-05-13  07:29               246 stdout.pl
2008-05-13  07:29                 0 stdout.txt
               2 File(s)            246 bytes

X:\>
[download]

(output trimmed to protect national secrets.)

To save/restore the parent's original STDOUT, just follow the examples briefly described by ysth that are more fully covered in that standard open documentation.