Check out Time::Duration

use Time::Duration;
print "Runtime ", duration(time() - $start_time), ".\n";
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If the time() - $start_time is 3820 seconds (which is exactly 1h, 3m, 40s), you get it rounded to fit within two expressed units: "Runtime: 1 hour and 4 minutes.". Using duration_exact instead would return "Runtime: 1 hour, 3 minutes, and 40 seconds".