my @hrefs = ...;
my %uniqHrefs; 

### This will create one entry in %uniqHrefs for each unique value in @hrefs;
@uniqHrefs{ @hrefs } = ();

my $res = Net::DNS::Resolver->new;  
 
foreach $urls ( keys %uniqHrefs ) {  
 
    my $query = $res->query($urls, "A");  
...