sub function_fast { ... }
sub function_full {
   ...
   function_fast( ... ); ## If possible, else copy-paste common code despite the DRY principle...
   ...
}
sub function;

sub function_init {
    my %args = @_;

    validate_args( \%args );

    if( added_functionality_required ) {
        *function = \&function_fast;
    }
    else {
        *function = \*function_full;
    }
}