A few more little suggestions about your code.

• You use both -w and use warnings. They both do pretty much the same thing so only one is needed.
• Your while condition can be better written as while (<FILE>).
• Rather than opening specific input and output files, your script would be more flexible if you read from STDIN and write to STDOUT. You could then call your script using IO redirection.
  myscript.pl < input.dat > output.txt

Perl Training in the UK <http://www.iterative-software.com>