Simply include in your log-in form a hidden parameter which tells the page to return to, and in your log-in program, send a redirect back to that page, either with the session parameter added or with a cookie set or however else you log in your users.

The form could look like the following (in Template::Toolkit syntax):

<form action="/login.cgi">
    <input type="hidden" value="[% returnurl %]" name="returnurl" />
    <input type="text" name="user" />
    <input type="password" name="password" />
    <input type="submit" value="login" />
</form>
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And the untested receiving code could be something like:

use CGI;

my $q = CGI->new();

if (is_valid_login($q->param('user'),$q->param('password')) {
    print $q->redirect($q->param('returnurl'));
} else {
    print "Forbidden";
};
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20060727 Fixed thinko in the code. s/redirecturl/returnurl/. Spotted by Hue-Bond, thanks!