Memory leak with shared memory

Hello,

I have a "big" problem with threads and shared memory.

Example :
#!/usr/bin/perl

use threads;
use threads::shared;

my $char_test : shared;
$char_test = "A";

for (my $i = 0; $i < 25; $i++)
{
    $char_test .= $char_test;
}

print "done\n";

sleep 5;

print "undef\n";
undef($char_test);
print "undef done\n";

sleep 60;

If I comment "use threads;", all is ok (memory usage is about 3.5% and
after undef, about 0.1%). But, if I uncomment "use threads;", memory usage
of perl is double (7%) and after undef, memory usage is 3.5%...
You can see that with "ps auxw".

It's a big problem for my project. Somebody knows this problem ?

Thanks,
Adrien