The rand function (on Win32) only produces values in the range 0 .. 32767*.

If you multiply that by any factor, you will get multiples of the base values. E.g.

#! perl -slw
use strict;
use Data::Dumper;

my %hash;
$hash{ int rand() * 65536 }++ for 1 .. 1e6;

print "$_ => $hash{ $_ }" for sort{ $a <=> $b } keys %hash;
__END__
0 => 26
2 => 30
4 => 26
6 => 32
8 => 33
10 => 23

...

65520 => 33
65522 => 28
65524 => 26
65526 => 24
65528 => 23
65530 => 26
65532 => 43
65534 => 26
[download]

Note how there are no odd values produced. The multiplier is a twice the base, so all you get are multiples of two.

If the multiplier is 3* the base, you only get powers of 3. And so on.

For non-exact multipliers, the pattern is less obvious, but there will still be (lots) of values that will never be produced.

Math::Random::MT has a much larger base, a huge periodicity, it's written in XS for reasonable speed and is available as a PPM from AS. Personally, I think it should have been included in Perl to replace CRT PRNGs years ago.

To quote the documentation for the rand function:

(Note: If your rand function consistently returns numbers that are too large or too small, then your version of Perl was probably compiled with the wrong number of RANDBITS.)