Sounds like an HTML5 app on the client side? You can use SQLite in the client for offline and also on the backend for your main app if so.

Something I have done for edit collisions is to send a digest of the record with the form. It becomes a "known last edit token."

In DBIx::Class/Catalyst, e.g., something like-

# Token is sent with form.
my $token = md5_hex(join("",sort $row->get_columns));

if ( $token eq $c->request->body_params->{token} )
{
    # Record hasn't been updated by anyone since the form was loaded.
    # Continue with edit/update...
}
else
{
    # Warn about edit collision, pretty format diffs/choices
    # with Algorithm::Diff/Algorithm::Diff::Apply.
}
[download]

As far as modperl, your deployment options will be limited. Unless you have an actual need of deep hooks in the request cycle + apache, there are "better" options today; nginx or lighttpd with Starman for example. I'd encourage you, strongly, to checkout Plack and the various web app frameworks like Catalyst, Dancer, and several other new ones.

Update: fixed syntax error in dummy code.