In perl, explicit code like this is not necessary as it would be in something more verbose like java or python. Perl has a concept of autovivification, where new values burst into being when needed. Also, ++ on a false value will return 1.

In the case where $hit[2] is not a key in %genus, requesting the key will create a corresponding value of undef. Incrementing this value will return 1.

so this snippet:

   #old snippet
   if (exists ( $genus {$hit[2]})) {   
       $genus{$hit[2]}++;                 
   }   else{                                        
           $genus{$hit[2]} = 1;           
   }
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can be reduced to this snippet:

#new snippet
$genus{ $hit[2] }++
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Example:

%genus = { } ; #genus is empty
$genus{ foo }; #requests foo entry, and throws away.
                      #genus is now { foo => undef }
$genus{ foo } ++; # genus is now { foo => 1 }
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http://en.wikipedia.org/wiki/Autovivification