The output should be sorted on lastname, followed by first name to break ties on lastnames.

There's a subtle bug in the sorting trick you used, sorting on "$last,$first". The inserted comma can throw off the sort, as in the following:

my @names = ("O'Toole,Peter",
             "O,Ari");
my @sorted = sort @names;
print join(";",@names),"\n";
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This prints:
O'Toole,Peter;O,Ari

To get the correct sort order, you need something like this:

my @names = (["O'Toole","Peter"],
             ["O", "Ari"]);
sub namesort {
    $a->[0] cmp $b->[0] or $a->[1] cmp $b->[1]
}
sub namejoin {
    return join(",",@{$_[0]});
}
my @sorted = sort namesort @names;
print join(";",namejoin($sorted[0]),namejoin($sorted[1])),"\n";
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My only comment on style is that you might use more subroutines; for example the median calculation would make a nice re-usable utility function.