in reply to how is this uninitialized?

$#downhosts++;
$#uphosts++;

[download]
You're altering the array size - increasing it by one. So, there's now an undefined value at the end of both your arrays.

Better would be: 

print "There are ", scalar(@downhosts), " hosts down\n";
print "There are ", scalar(@uphosts), " hosts alive\n\n";

[download]
and remove the $#uphosts manipulation.

Being right, does not endow the right to be rude; politeness costs nothing.
Being unknowing, is not the same as being stupid.
Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence.
Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.

In Section Seekers of Perl Wisdom