$dir='path to directory';
opendir D, $dir or die "Cannot open directory $dir: $!\n";
while( defined (my $file = readdir(D)) ) {
        if ($file =~ /\.dat$/){
                push(@files,$file);
        }
}
closedir D;
@files = sort @files;
@filename = split(/\./,$files[$#files]);
print $filename[0],"\n";
$filename = ($filename[0] + 1) . '.dat';
print $filename,"\n";