You probably want something like:

   if ($species eq "H") 
   {
     require human;
     human->import();
   }
   elsif ($species eq "M") 
   {
     use mouse;
     mouse->import();
   }
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The reason that what you had didn't work is that the use happens are compile time before the code is executed.

You might want to consider using proper cased names for your modules as all lowercase names are conventionally reserved for pragmatic modules such as strict

/J\