switching between insert and update

Not sure if this is the right list but here goes:

I've got a perl script that inserts some data.  Simple enough, but my
problem is that it fails when I try to insert more data that contains the
same time and id.  Is there an easy way of switching from an INSERT to an
UPDATE and does anyone know of any examples?  There's one more piece to
this puzzle.  The columns names and number of columns addressed are
dynamic and will change for each file loaded into the database.

Thanks,

Travis