Default, perl treat each line as a record, but you can tell perl to change it's notion of what a record is:

#!/usr/bin/env perl
use strict; use warnings;

use English;
$INPUT_RECORD_SEPARATOR = "\n\n";

while (<DATA>){
    s/\n/ /g;
    print $_, "\n"
}
__DATA__
Start: afadffdafadf
stuff
stuff
stuff
stuff
Close: xzy

Start: afadffdafadf
stuff
stuff
stuff
stuff
Close: xzy

Start: afadffdafadf
stuff
stuff
stuff
stuff
Close: xzy
[download]