use threads ();
my $thread;
BEGIN {  # execute this at compile time
    $thread = threads->new( sub {
        print "Benchmark has not been loaded!\n" unless defined $Bench
+mark::VERSION;
        # do your Benchmark stuff
    } );
}
use Benchmark;
$thread->join;
[download]

which prints:

Benchmark has not been loaded!
Scalars leaked: 1

Yikes! What is that! "Scalars leaked: 1". Well, yes, that's one of the remaining problems/features/bugs of the Perl ithreads implementation. This particularly seems to happen when you start threads at compile time. From practical experience, I must say it seems to be pretty harmless.

The leaked scalar is the result of returning a closure from a thread. In many cases it is, as mentioned, harmless. However, in some implementations of Perl, returning a closure from a thread may cause the interpreter to crash (i.e., dump core). Caveat emptor.