Thanks, tye, have this all figured out now.

fd_pass.pl

#!/usr/bin/perl
use v5.14;
use warnings;
use Win32API::File 'FdGetOsFHandle';


my $FILE = shift or die "Need file to read\n";


say "Opening file";
open( my $in, '<', $FILE ) or die "Can't open '$FILE': $!\n";

# Get the real FD from Windows
my $fd = FdGetOsFHandle( fileno($in) );

say "Spawning child process";
my $pid = system(1, 'perl', './fd_get.pl', $fd )
    or die "Could not spawn child process: $!\n";

wait;
[download]

fd_get.pl

#!/usr/bin/perl
use v5.14;
use warnings;
use Win32API::File 'OsFHandleOpen';

my $FD = shift or die "Need file descriptor\n";
OsFHandleOpen( *IN, $FD, 'r' )
    or die "Could not open file descriptor '$FD': $!\n";


while(<IN>) {
    chomp;
    say "Got in child: $_";
}

close IN;
[download]