tried it out on the ancient linux box (perl 5.8.5)...

how can I get around this warning?

Upgrade. The version of perl producing the warning is 12 years old which is positively triassic in computing terms. I don't have an installation that old to test it on, but your commands with ->detach work silently on 5.8.8 which is the oldest I have to hand (and is still a decade old).

$ perl -v

This is perl, v5.8.8 built for x86_64-linux-thread-multi

Copyright 1987-2006, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5 source ki
+t.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using "man perl" or "perldoc perl".  If you have access to
+ the
Internet, point your browser at http://www.perl.org/, the Perl Home Pa
+ge.

$ perl -Mthreads -le 'async { sleep 100; }->detach ; exit;'
$
[download]