One way is to get a list of the matching files in the directory, sorting them and adding to the digit. This example outputs last digit and the new filename to STDOUT to demonstrate:

(remember to use strict when programming)

$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";
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Mick