open my $infile, "<", "test.txt" or die "cannot open input: $!";
my @infile1= <$infile>;
close $infile;

foreach my $dns ( @infile1 ) {
    chomp $dns;
    if ($dns =~ /(dns:[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})$
+/){
    my ($ip, $dns1) = split /dns:/, $dns;
    ($ip) = split /,/, $dns1;
    print "$ip";}
}
[download]

Perhaps you want something like this:

open my $infile, "<", "test.txt" or die "cannot open input: $!";

while ( my $dns = <$infile> ) {
    if ( $dns =~ /dns:([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3})
+/ ) {
        print "$1";
    }
}
close $infile;
[download]