.i mean what basis it is comparing with the values

On the basis you specify in your sort subroutine (in this case, numeric_sort).

actually arguments are stored in @_ inside subroutine

Sorting subroutines are special, for historical reasons.

how does $a and $b gets the value

Perl's sort builtin takes care of that, automatically.

is it like bubble sort

No, something rather faster. I think thought it might be a heapsort, but I could be was misremembering, or and it might have changed at some point. In any event, that's an implementation detail of the type you shouldn't worry about. The sort is one of the pieces of perl that's fairly well optimized, so unless you're doing something rather unusual, it will generally perform rather better than any sort you can code by hand. The exception is when the comparison operation itself is very expensive (e.g., because it involves a lookup over a slow network), in which case a common optimization called a Schwartzian Transform can be used to perform the expensive part of the operation only once per each item in the list.