Well, I'm wondering if it might be easier for you to use
a database, as opposed to files. What you could for every
classified ad is have a field for (amongst other things)
the ad itself, and the day it was posted. Whenever it is
posted, just use Perl to generate a datestamp and put it
into that database field. Then every evening, you could
have a cron job to dig through the database and expire (delete?)
entries that are 7 days old.

To use a database this way, you wouldn't even be required to
have a full fledged database server running in the background,
you could use something as simple as CSV files if thats what
you want. OTOH, MySQL and Postgres come pretty cheap :-)
-Eric