I'm not sure if any of this is helpful as I'm on a different platform and I'm not at all sure that anything I'm seeing will be applicable. When I run this version of your code here:

#! perl -sw
use 5.010;
use strict;
use threads;

$_->join for map {
    async {
        for( 1 .. 7e6 ) {
            int rand 10;
        }
    };
} 1 .. 64;

say "Test 1 complete"; <STDIN>;

$_->join for map {
    async {
        my $tmp;
        for ( 1 .. 7e6 ) {
            for ( 1 .. 1e5 ) {
                $tmp = $_;
            }
            int rand 10;
        }
    };
} 1 .. 64;
[download]

I do perceive a very slight drop in cpu sctivity between the first batch and the second.

The first nails the cpu counter at (or very close to) 75% (100% of 3 of 4 cpus--I have something running on the other cpu). What minor wobbles occur go no lower than 74.8; and it occasionally shows slightly greater than 75%--though that probably just the result of maths.

The second again gets close to 75%, but the wobbles are larger. It occasionally drops as far as 73% and rarely get as high as 74.8%. So something is preventing the second from maxing the cpu. I do not see any signs of locks, though I may not be instrumenting the right things. I need to look further into the measurements available--the list is very long. But in any case, the drop is nothing like as dramatic here, as your instrumentation appears to show on your platform!

Following anonymonks lead, I replaced (both) implicit loop counters with lexicals in the second batch:

        for my $i ( 1 .. 7e6 ) {
            for my $j ( 1 .. 1e5 ) {
                $tmp = $j;
            }
[download]

And the drop in cpu utilization "went away", which does tend to indicate that it is somehow related to use of the global $_. Whilst that is global, it isn't shared, (I bleieve it is cloned on a per interpreter basis), so it shouldn't require internal locking. But globals do carry a small penalty over lexicals--regardless of the use of threads--so whether that has anything to do with it I am unsure. I can't see why it would, but I have considerable difficulty follwing the internals in areas that relate to cloning.

I'll try too look into it in more detail, but that will take a while.