Got some interesting (to me) data on this comparison. After finding a way to track memory and cpu usage for child processes, here's how threads vs forks stacks up:

threads.pm:
WC: 13.76s  Usr: 0.39s  Sys: 0.07s  
CSv/f: 0/0  IOops: 0/0  Sigs: 0  Swaps: 0  PF: 2500/1265  Msg: 0/0  Me
+m: 65499  

forks.pm:
WC: 18.30s  Usr: 0.33s  Sys: 0.22s  
CSv/f: 0/0  IOops: 0/0  Sigs: 0  Swaps: 0  PF: 14427/2524  Msg: 0/0  M
+em: 5454
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These come from running the exact same tests ~20 times, varying only the use of threads.pm vs forks.pm. The code for this test is below, it does do a little bit of data sharing (since my intended usage model will as well).

Interesting to note that forks.pm does use far less memory, but gets far more page faults, and also uses more cpu time. Guess it depends which is more valuable to you. As stated by others above, forks.pm isn't necessarily the ideal implementation for using forks, but rather a convenient one because it uses the threads.pm interface.

Here's the code I used. For both threads and forks, I ran 9 times with 10 threads, 6 with 20, 3 with 50, and 3 with 100.

# which threading model do we want?
use threads; use threads::shared;
#use forks; use forks::shared;


$num_threads = shift(@ARGV) || 10;

$nonshared_var = 1;
my $shared_var : shared = 1;

foreach (0..$num_threads-1) {
    # create a new thread
    push(@threads,threads->new(\&thread_sub));
}

foreach $thread (@threads) {
    # wait for threads to finish
    $thread->join();
}

printf "Orig thread done\n";

sub thread_sub {
    my($count) = 20*rand() + 1;
    my($random);
    printf "Thread %d started\n",threads->tid();

    # do some stuff
    while ($count-- > 0) {
        $random = (5*rand()) | (time() & 0x3);
        $nonshared_var += int($random);

        lock($shared_var);
        $shared_var += int($random);

        sleep($random);
        printf("  Thread %d loop %d: random=%d, nonshared_var=%d, ".
               "shared_var=%d\n",threads->tid(),$count,$random,$nonsha
+red_var,
               $shared_var);
        threads->yield();
    }
    printf("Thread %d DONE!\n",threads->tid());
}
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