If the I/O isn't your problem then consider the following:
* Don't use Perl. Re-write this in C, keep your keys in a tree of some kind (determined by the distribution of the keys) and the values (tot, max, last) in the tree. It's always faster to use a specifically designed tool (a single-purpose, well written C program) than a general purpose multitool (like Perl).
* If you have any control over the input stream: instead of tab-delimited data, use space/column-delimited data. It's (probably) faster to extract two substrings of known width than to split. Leave the whitespace on the keys when storing, remove it later when redisplaying (if necessary).
