We are trying to open a file descriptor and access it from a forked + exec child.

Here's a working SSCCE counterexample. It uses the PPID to retrieve the correct FD.

#!/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;
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Here's the simpler one for not shelling out:

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

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

unless (fork) {
    print "CHILD: $_", for grep { /root/ } <$in>;
    print "CHILD: we're done here\n";
    exit;
} else {
    print "PARENT: still going\n";
    wait;
}

close $in;
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