I personally use Time::Stopwatch for this. If you use Time::HiRes as well, it will be more precise. The code is clean and easy to read:

use Time::Stopwatch;

tie my $timer, 'Time::Stopwatch';

#.. your code here

print "Completed in $timer seconds";
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Of course, you can use some simple math to break the elapsed seconds into H:M:S format if you'd like.