http://paperlined.org/dev/src/pl/tmp/test.pl

#!/usr/bin/perl

# figure out if file handles are shared properly across fork()
#
## AHA!  It works!!!!  The difference is between buffered- and non-buffered-IO

    use strict;
    use warnings;

    use Data::Dumper;


my $scalar;



# http://search.cpan.org/dist/perl/pod/perlfunc.pod#tell
#   There is no systell function. Use sysseek(FH, 0, 1) for that.
#
# More on buffering:
#   http://perl.plover.com/FAQs/Buffering.html
#

open FH, "test"    or die;


print "[1] tell=" . sysseek(FH,0,1) . "\n";
my $pid = _fork(sub{
    sleep 1;
    print "[3] tell=" . sysseek(FH,0,1) . "\n";
    sleep 2;
    print "[5] tell=" . sysseek(FH,0,1) . "\n";
});
my $ret = sysread(FH, $scalar, 5);
print "sysread returned $ret\n\tcontents = $scalar\n";
my $ret = sysread(FH, $scalar, 5);
print "sysread returned $ret\n\tcontents = $scalar\n";
print "[2] tell=" . sysseek(FH,0,1) . "\n";

sleep 2;
sysread(FH, $scalar, 20);
print "[4] tell=" . sysseek(FH,0,1) . "\n";


waitpid($pid, 0);




sub _fork {
    my ($child_func) = @_;
    defined(my $pid = fork) or die "Unable to fork: $!";
    if ($pid == 0) {
        $child_func->();
        exit;
    } else {
        return $pid;
    }
}