On win, I see a slow creep up of memory usage for the first few thousand iterations, but it then stabilises and stays rock steady for tens of thousands more:

#!/usr/bin/perl
use Thread;
use Time::HiRes qw(usleep);

my $count = 0;
while( 1 ) {
    usleep(100);
    my $thr1 = new Thread \&doNothing;
    $thr1->join;
    unless( ++$count % 1000 ) {
        printf "%3d: %s", $count,
        ( qx[tasklist /fo list /fi "pid eq $$"] )[ 5 ];
    }
}

sub doNothing {}
__END__
C:\test>junk
1000: Mem Usage:    11,576 K
2000: Mem Usage:    11,752 K
3000: Mem Usage:    11,880 K
4000: Mem Usage:    11,944 K
5000: Mem Usage:    12,544 K
6000: Mem Usage:    12,544 K
7000: Mem Usage:    12,544 K
8000: Mem Usage:    12,544 K
9000: Mem Usage:    12,544 K
10000: Mem Usage:    12,544 K
11000: Mem Usage:    12,544 K
12000: Mem Usage:    12,544 K
13000: Mem Usage:    12,544 K
14000: Mem Usage:    12,544 K
15000: Mem Usage:    12,544 K
16000: Mem Usage:    12,544 K
17000: Mem Usage:    12,544 K
18000: Mem Usage:    12,544 K
19000: Mem Usage:    12,544 K
20000: Mem Usage:    12,544 K
21000: Mem Usage:    12,544 K
22000: Mem Usage:    12,544 K
23000: Mem Usage:    12,544 K
24000: Mem Usage:    12,544 K
25000: Mem Usage:    12,544 K
26000: Mem Usage:    12,544 K
27000: Mem Usage:    12,544 K
28000: Mem Usage:    12,544 K
29000: Mem Usage:    12,544 K
30000: Mem Usage:    12,544 K
31000: Mem Usage:    12,544 K
32000: Mem Usage:    12,544 K
33000: Mem Usage:    12,544 K
Terminating on signal SIGINT(2)
[download]

If there's a leak, it appears to be platform specific.