A problem with the above is that after the first valid IP address has been defined, there is nothing that explicitly undefines it, so the following line becomes useless:

next unless defined $ip;

The minimal change you could make to fix this bug would be to add $ip = $classc = undef; just before the if block.

You could improve it further by use strict combined with putting a bit of thought into the scoping of variables.

It can be reduced to:

use 5.010;
use strict;
use warnings;

my %ips;
while (<$in_fh>)
{
    if (/^(\d+)\.(\d+)\.(\d+)\.(\d+)/)
    {
        my $ip     = "$1.$2.$3.$4";
        my $classc = "$1.$2.$3";
        push @{ $ips{$classc} }, $ip;
    }
}

foreach my $classc (sort keys %ips)
{
    say $ips{$classc}[rand @{$ips{$classc}}];
}
[download]

Personally I'd go further and reduce it even more. This might be a little too terse for some people's tastes though...

use 5.010;
use strict;
use warnings;

my %ips;
while (<$in_fh>)
{
    push @{ $ips{"$1.$2.$3"} }, "$1.$2.$3.$4"
        if /^(\d+)\.(\d+)\.(\d+)\.(\d+)/;
}

say $ips{$_}[rand @{$ips{$_}}]
    for sort keys %ips;
[download]