The proper end index is int rand @array, not int rand $#array. For example, for an array of 7 items, rand @array would return a number that is equal to or higher than 0 or, and below 7. That means that the largest number you can ever get would be like 6.9999999999.... int truncates this to an integer towards zero (AKA "rounding down"), so the largest integer you can ever get this way, is 6.

What's more, rand is uniformely distributed, which means that the chance of getting an outcome within any subrange is proportional to the size of that subrange, thus, the difference between upper and lower limit of the subrange — the lower edge is included in this range, the upper edge is not. Therefore, after int, the chance of getting any integer in the range is the same for all, as the size of each subrange resulting in one particular integer value is 1 — including the last one.