Following What XML generators are currently available on PerlMonks?, you may want to use a URL like:

http://www.perlmonks.org/?node_id=30175;types=perlquestion;days=0.0034
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[download]

This will only look for nodes created in the last 0.00347 days, which is 5 minutes (the amount of time your script sleeps between polls). It's a little unnecessary load on the server, perhaps, to fetch the information for the older irrelevant nodes. Also, it eliminates the need to store a "last seen" node ID.

I also notice that your script will not announce more than one node if multiple new nodes show up in the same 5 minute window.