Pass a file descriptor from a process to another process through an UNIX socket, with Socket::MsgHdr.

fdpass.pl

#!/usr/bin/env perl

use strict;
use warnings;

use Socket;
use Socket::MsgHdr;
use IO::Handle;

my ($parent, $child);
socketpair($parent, $child, AF_UNIX, SOCK_STREAM, PF_UNSPEC);

if (fork) {
    close $parent;
    open my $fh, ">>/tmp/fugafuga" or die;
    my $fd = fileno $fh;
    my $outHdr = Socket::MsgHdr->new(buflen => 8192);
    $outHdr->cmsghdr(SOL_SOCKET, SCM_RIGHTS, pack("i", $fd));
    print "Parent: $fd\n";
    sendmsg($child, $outHdr);
    close $fh;
    wait();
} else {
    close $child;
    print "CHILD\n";
    my $flags = 0;
    my $inHdr = Socket::MsgHdr->new(buflen=>8192, controllen=>256);
    recvmsg($parent, $inHdr, $flags);
    my ($level, $type, $data) = $inHdr->cmsghdr;
    my $fd = unpack("i", $data);
    print "Child: $fd\n";
    my $fh = IO::Handle->new;
    $fh->fdopen($fd, "a");
    $fh->autoflush;
    print $fh "test\n";
    print "CHILD DONE\n";
}