slow update & index question

I have a script that is updating the DB by reading in a file with 467 lines. Each file is named by the shelf the information was gotten from. Each line is tab seperated and consists of a column name and the value that it should be update to. The update line in the perl script looks like this: update table set "column name" = 'value' where "Shelf_Name" = '$shelf'

Now this works.. but it takes almost 1 minute to do the updates. With nothing else at all touching the DB.. is this normal? The DB is index'd on the Shelf_Name column.. but how do I know if it is using the index.. or does that not even matter in this case?

Travis