forking, and then calling system to run the command, means he's forking twice for every command. Maybe 3 times depending what's in $command.

You can achieve the goal without threads on win32 like this:

#! perl -slw
use strict;

my @dates = 1 .. 10;
my  @pids;

(my $cmd = <<'CMD' ) =~ s[\s+][ ]g;
    -wle"print qq[task $ARGV[0] starting];
      sleep rand( 10 );
      print qq[task $ARGV[ 0 ] finishing];"
CMD

while( my $date = pop @dates ) {
    push @pids, system 1, 'perl.exe', "$cmd $date";
    if( @pids >= 5 ) {
        my $kid = wait;
        @pids = grep $_ != $kid, @pids;
    }
}

waitpid $_, 0 for @pids;
[download]

Since you need to load a new executable, why not just start it in a new process rather than duplicating the current one first?