I'm thinking of using cron and reading the files' dates

That's exactly what I do. Works like a charm.

Sample code:

    my $tmp_path = "/tmp/myapp";
    my $max_age  = 60*60*24;    # e.g. one day

    my $t_keep = time() - $max_age;
    my @stale_sessfiles =
        grep { (stat $_)[9] < $t_keep }  # files older than $t_keep
        glob "$tmp_path/cgisess_*";

    # print STDERR "$_\n" for @stale_sessfiles;  # debug

    unlink @stale_sessfiles;
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