If your intention is to save memory, you can avoid the duplication, at the cost of more run time:

my %mirror = (
    'http://foo.mirror.org'                    =>   3,
    'http://www.users-domain.com/proj/foo'     =>   2,
    'http://osdn.dl.sf.net/sourceforge/foo'    =>   3,
    'http://unc.dl.sf.net/sourceforge/foo'     =>   4,
    'http://umn.dl.sf.net/sourceforge/foo'     =>   4,
    'http://heanet.dl.sf.net/sourceforge/foo'  =>   5,
    'http://aleron.dl.sf.net/sourceforge/foo'  =>   5,
);
my $t = 0;
my $choice;
while (my ($url, $n) = each %mirror) {
    $choice = $url if rand ($t += $n) < $n
}
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Of course, in real code, this is only worthwhile if your numbers are high, and the number of mirrors isn't.

Abigail