Using gettimeofday and tv_interval makes it a bit simpler.

$ perl -Mstrict -Mwarnings -MTime::HiRes=gettimeofday,tv_interval -E '
my $t0 = [ gettimeofday() ];
my @psLines = qx{ ps -ef };
print
   qq{Command took },
   tv_interval( $t0, [ gettimeofday() ] ),
   qq{ seconds to run\n};'
Command took 0.015902 seconds to run
$
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I hope this is helpful.