How could I insert two tabs before each Description so that the tabs are shown in the html-output?
=pod
=head2 Head 2
My list:
=over
=item
First Description A
=item
Second Description B
=item
Third Description C
=back
=cut
Do you mean how to make a definition list?
Try:
=over
=item First
Description A
=item Second
Description B
=item Third
Description C
=back
That will format a definition list with "First", "Second", and "Third" as the terms and "Description A", "Description B", and "Description C" as your descriptions.
If you need to change the formatting of a definition list on search.cpan.org or metacpan.org, you cannot. POD is a semantic documentation language and doesn't give the module author very much control over the layout and typography. The web site owner is the one that has the power to adjust the typography and such via CSS.