If you are expecting the statement ... to interchange the elements, you are missing a step or two.

No, that part works fine:

my @usernames = qw(zero one two three four five six);
testswap(\@usernames);
print join(" ", @usernames), "\n";

sub testswap {
 my $array = shift;
 my $i = 2;
 my $j = 5;
 @$array[$i, $j] = @$array[$j, $i];
}
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Prints: zero one five three four two six

As runrig pointed out, you're not returing a value, you could put return $array->[0] at the end of the subroutine.

If you just want to pick one element out of the list, you're going to too much trouble. You can just say:

my $username = $usernames[rand(@usernames)];
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