A shopping cart application almost certainly needs a "back end"
database to store product and inventory information, as well as
transaction records of purchases, etc. There are several
options:
For "small scale" operations, this can be done using simple
text files. Many Unix utilities are designed to manipulate data
stored in files of this type, and Perl is very well suited to the
job. These utilities can be "glued" together to build quite
advanced systems.
If the task is sufficiently large or complex, there are many
"low end" cheap, or free, database solutions. In this case, the CGI
program interfaces to the database, normally by sending SQL
queries. The standard Perl database access module is
DBI::, which interfaces to virtually all DMBSs in
a standardises way. The current defacto Web DBMS is probably
mySQL, but lots of others are available.
Proprietory databases such as Microsoft "Access" can be used in
conjunction with an associated proprietory Web server such as IIS.
Perl's CBI:: can also talk to most of these.
At the "big end" of town, companies like Oracle are positioning
themselves as major Web database vendors.