You aren't clearing $sum at the beginning of each outer loop. Try this:
#!/usr/bin/perl
use strict;
use warnings;
my $sampleSize = 7;
my ($sum, $ssum);
for my $i (1..1000){
$sum = 0;
for my $j (1..$sampleSize){
$sum+=rand();
}
$sum/=$sampleSize;
$ssum+=$sum;
}
$ssum/=1000;
print "$ssum\n";
If you think you've found a show-stopping bug in a core function that others have been using successfully for years or decades, it's a good idea to examine your own code before declaring it a bug in Perl.
Edited to add: Besides, how could the rand() function "know" how many times you're about to call it, or how many you're not going to call it? Short of a maliciously buggy dev-teasing version of Perl, which is too far-fetched to consider, there's no possible mechanism that could do this.
