#!/usr/bin/env perl 
use strict;
use warnings;
use autodie;

$| = 1;
open my $in, '<', '/etc/passwd';
my $ppid = $$;

unless (fork) {
    print "CHILD: ";
    exec ("grep", "root", "/proc/$ppid/fd/" . fileno($in));
} else {
    print "PARENT: still going\n";
    wait;
}

close $in;