When you use the line:
open (DAT, "+>>$file") or die "$!";
You aren't just opening the file read/write, you're opening it read/write in *append* mode. That means the pointer is at the end of the file when you open it. So when you read it, you don't get anything. There are a few things you can try here to learn a little more. Here's a little debugger session I played with that might help you get this working correctly. There's also some other things I do differently here that you may wish to adopt as standard practices, such as using a lexical filehandle and 3-argument form of open:
DB<1> open $fh, '+>>', 'test.dat'
+
+
DB<2> if ( eof $fh ) { print "Don't try to read, you're at the end!\
+n" }
+
Don't try to read, you're at the end!
DB<3> seek $fh, 0, 0
+
+
DB<4> if ( not eof $fh ) { print "Read on, McDuff!\n" }
+
+
Read on, McDuff!
DB<5> @lines = <$fh>
+
+
DB<6> x @lines
DB<7> chomp @lines
+
+
0 'First Line'
1 'Second Line'
2 'Last Line'
Hope this helps,
~Jeff
