Hello abhishekv,

what is the reason behind it

It's a useful function, often needed, and other languages of the era did not often provide this function built in to the language. Larry had a flash of brilliance and said -- hey, here's a cool feature, and added it to the definition of Perl.

i want to know the internal functionality for this case

It isn't hard to imagine. Either:

• Perl tracks how many elements are in each array and that value is returned, or;
• Perl doesn't track how many items are in the array, so it loops through all the entries and counts them, then returns that value.

My money is on the former. I'd bet a whole quarter on it.