My guess is that it's due to how fork is implemented. Perhaps try Win32::Process::Create().

Update: This leaks too (under AS perl 5.10):

Another update: I take that back...it grows to some size and then remains around the same.

#!/usr/bin/perl

use strict;
use warnings;


my $i;
while (1) {
  unless ( ++$i % 20 ) {
    print "$i: ";
    my $p = scalar(<STDIN>);
    while (1) {
      my $pid = wait;
      print "[$pid]\n";
      last if ! $pid or $pid == -1;
    }
  }
  my $pid = fork();
  die "No fork: $!" unless defined $pid;
  print "$pid\n" if $pid;
  next if $pid;
  exit;
}
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