thread: avoid Perl5 internal scratchpad target cache

thread: avoid Perl5 internal scratchpad target cache

The use of array-returning built-ins such as `grep' inside
arrayref declarations appears to result in permanently allocated
scratchpad space for caching according to my malloc inspector.

Thread skeletons get discarded every response, but multiple
skeletons can exist in memory at once, so do what we can to
prevent long-lived allocations from being made, here.

In other words, replacing constructs such as:

	my $foo = [ grep(...) ];

with:

	my @foo = grep(...);

Seems to ensure the mortality of the underlying array.