ofcourse using regex alone you can get your desired output, but since you said you are studying regular expression with splitting, maybe the code below could help:

use warnings;
use strict;

my %hash;
while(<DATA>){
  chomp; 
  my ($dns)=grep $_=>split/^.+?:/,$_;
  my ($ip,$dns1)=split/,/,$dns;
   $hash{$dns1}=$ip;
}
 print $hash{$_},$/ foreach keys %hash;

__DATA__
host1    dns:192.168.243.30,asdf
host2    dns:192.168.243.1,qwert
[download]

You could check perldoc -f split for details
Hope this helps