I'd try getting better feedback from your script, in production mode, when the failure occurs, using Data::Dumper, Carp etc to report verbosely about your code's state. If you can't provoke the error at will, you need to get as much info as possible when it does happen.

Also, trap and test the return code of everything - and I mean everything.

Try writing your own signal handlers too, if that seems appropriate - stuff like

 $SIG{'INT'} = &int_handler;

sub int_handler {
...verbose state printing here...
}
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I have already run it with the debugging turned off. There was no change in the behavior. The same script on another machine works with it set to maximum.

Calling sync() before bind() and search() produces: Can't use an undefined value as a HASH reference at /usr/share/perl5/Net/LDAP.pm line 696, <DATA> line 225.

The docs say that sync() should accept null arguments....

Could you produce and post a small sample program that exhibits this behaviour?

From looking at the source, I believe that $ldap->{net_ldap_mesg} does not exist/is not filled/is undef. Whether this makes sense when you call $ldap->sync(), I don't know, but it would help in debugging the error. Maybe you need to call some other method before you can call sync()? For example if you have no outstanding messages?

can't help you too much without seeing any code

Update: Logic errors maybe present in your code which no debugger will pick-up can't tell if we can't see the code

Code or no code, the fact remains that the script behaves differently when run in the debugger.

#!/usr/bin/perl  -w

use Net::LDAP;
use strict;

my $ldap = Net::LDAP->new(
        "ldap://server/",
        debug=>15,
        version=>3,
        async => 1 ,
        onerror => 'warn'  );
die "new(): $@" if !$ldap;


my $result = $ldap->bind();
die "bind(): $@\n" if  $result->code;

$result = $ldap->search(
        base => "dc=zzzz,dc=com",
        filter => "(&(objectclass=posixAccount) )",
        attrs=>['uid'] );
die "search(): $@\n" if $result->code;

print "Results:\n";

foreach my $entry ( $result->entries) {
        $entry->dump;
}

$ldap->unbind();
$ldap->disconnect();
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Update

Yes, I tried standard debugging techniques. The LDAP bind() and search() are where it fails.

An example of how random it is(with Net::LDAP debugging off):

emeitner@brule:~/work/perl-ldap$ while : ; do ./simple.pl ; sleep 1; d
+one
bind(): I/O Error Resource temporarily unavailable
bind(): I/O Error Resource temporarily unavailable
bind(): I/O Error Resource temporarily unavailable
search(): I/O Error Resource temporarily unavailable
bind(): I/O Error Resource temporarily unavailable
bind(): I/O Error Resource temporarily unavailable
search(): I/O Error Resource temporarily unavailable
bind(): I/O Error Resource temporarily unavailable
bind(): I/O Error Resource temporarily unavailable
search(): I/O Error Resource temporarily unavailable
bind(): I/O Error Resource temporarily unavailable
search(): I/O Error Resource temporarily unavailable
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I DID notice during my investigation that if I use other modules such as Data::Dumper the script will actually work once out of every 20 runs or so.

Yep, I tried that too. It behaves the same in either sync or async modes.

Ok... Try putting an explicit $ldap->sync() before your bind() and search() calls.