Basically what Chromatic and Gaal have already told you, but also:

use vars qw/ %opt /;
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Why not just use sub-quotes instead? You're quoting a word, not words.

sub revlookup
{
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revlookup? What does that mean? I'm gonna take a guess and say "reverse lookup," but the only way I could determine that was to look at the routine code itself. I shouldn't have to do that. The name should tell me exactly what the routine does. Why not reverse_lookup() or--if you like CamelCase--reverseLookup() instead?

my $res=Net::DNS::Resolver->new;
$res->nameservers($server);
my $search = $res->search($input);
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Inconsistency does terrible, terrible things to software. How come some of your assignment statements have spaces on either side of the equals sign but others don't?

if ($search)
{
    foreach $rr ( $search->answer)
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Again with the inconsistancy. Why the extra space around $search->answer but not $search? They're both conditionals, right?

sub fwlookup
{
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The name thing again: try forward_lookup() or forwardLookup() instead.

sub options
{
    use Getopt::Long;
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That use statement belongs towards the top of the script; it's a compile-time declaration and in this case sticking it inside of a routine doesn't change its behavior, it merely obfuscates it.

I would recommend giving perlstyle a good once-over. You don't have to take it as gospal, but at least consider each and every one of its recommendations and follow them unless you have good reason not to.