Dangerous (be sure to have an independent backup before you read on):

If you want to edit the data in place, you could use

perl -ibak -we 'local $/ = "\n\n"; while (<>) { local $/ = "\n"; chomp
+; print }' ./data.dat
[download]

Be aware that it'll apply the changes immediately, but will create a backup of the file edited with the extension .bak. Furthermore, this approach assumes that your _entire_ input file looks like the excerpt you provided. Bear in mind that otherwise perhaps, chaos will ensue.