If I understand the description, you want a boolean value for each date+state tuple -- that is: "there (was | was not) at least one tornado on date X in state Y".

If that is the case, think about creating an array of 50 elements (one for each state). The value of each element is a reference to a hash, where the keys of the hash are the dates when one or more tornados occurred in the given state (the value assigned to each element is of no importance: it is the existence of a given date as a hash key for the given state that matters). So, suppose the data is like this:

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
...
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You might try something like this, just to list how many tornado days there were in each state, based on the given input report:

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[$_]} );
   }
}
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(untested, of course)