That's exactly what I did just recently:

use Time::HiRes;
...
my $start = Time::HiRes::time();
...
do your job
...
my $stop = Time::HiRes::time();

my $duration = $stop-$start;
[download]

It gives you the difference with a lot more precision that whole seconds, but if that's what you care about it should eb easy to round the number.