Why doesn't the beginning of the month in the Islamic calendar coincide with my perl calculations of the new moon?

Internet tells me that Gregorian date 19th of July, year 2023 is the first of Muharram. But when I try to calculate the moon phase via perl, (module Astro::MoonPhase), it tells me that on sunset with coordinates for Mecca ($latitude = "+21.422510"; $longitude = "+39.826168";), the moon was already more than half a day old at sunset on **18th** of July. What's wrong? Are the results from the computation wrong, or am I missing something in the interpretation of the computation? (Maybe both?)