perlmod has a nice description of how your modules can play nice with threads.
It describes the CLONE_SKIP routine which you can add to your module to prevent cloning across threads.

If you only want to not clone one of many instances, you could create a subclass Class::Foo::NoClone which is empty save for a sub CLONE_SKIP { 1 } and a @ISA = ('Class::Foo'); and rebless the specific instance into that.

Also, are you sure that it won't clone the second $x? Its been a while since I've used threads. Where in the docs does it say that? I would assume they would be treated equally, both being lexical variables and all.

Update: proof that the second $x is cloned 
use threads;
use strict;
use warnings;
use PadWalker qw(peek_sub);


sub thr {
  my $xref = peek_sub(\&run_test)->{'$x'};
  print "\$xref in thr: $xref\n";
}

sub run_test {
  my $x = 1;
  my $xref = \$x;
  print "\$xref before thr: $xref\n";
  threads->create(\&thr)->join;
  $xref = \$x;
  print "\$xref after thr: $xref\n";
}

run_test;

[download]
Output (on perl v5.8.8 built for x86_64-linux-gnu-thread-multi): 
$xref before thr: SCALAR(0x6c51a0)
$xref in thr: SCALAR(0x7584a0)
$xref after thr: SCALAR(0x6c51a0)

[download]

In reply to Re^3: Protecting External Data?! by plobsing
in thread Protecting External Data?! by semuel

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