State  Date        other stuff
1      03/03/2003  one bad mother of a storm...
5      03/05/2003  nothing to write home about...
8      02/08/2003  they said it could never happen here
...

##</code><code>##

my @states; # array holding one hash per state

while (<>)
{
   my ($state_number,$date) = split;
   $states[$state_number]{$date} = undef;
}

# now to summarize:
# for each state that had any tornados at all,
#    list number of tornado days:

for ( 1 .. 50 ) {
   if ( defined $states[$_] ) {
      printf( "state_id %2d: %3d tornado days\n", $_,
              scalar keys %{$states[$_]} );
   }
}