Don't hardcode perl path but use env instead

Don't hardcode perl path but use env instead

Also replace the deprecated "-w" argument with "use warnings;", as
otherwise env would invoke a command called "perl -w".

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Clemens Fruhwirth <clemens@endorphin.org>
Closes #5552