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;