The if.pm module is useful for that,

use if $species eq 'H', 'human';
use if $species eq 'M', 'mouse';
[download]

Make sure that $species is set at compile time (that's probably why it's being ignored).

This looks like a natural place for inheritance, though. Let Human.pm and Mouse.pm inherit from Mammal.pm, which carries the common subroutines.