Given that you are always using dosumpton, it would seem more logical to me to explicitly treat an argument list and pass that to your subroutine. Is there some reason you need to explicitly pad your argument list to 4 terms? If not, then your script is functionally very close to:

if($mystring){
    dosumpton(split(/\./, $mystring));
} elsif (not defined $mystring){
    dosumpton("nogames");
} else {
    # Something went wrong and we should be notified about it
}
[download]

The way in which these two forms differ is the question of trailing false but defined values - zeros ('0') and empty strings (''). Your clobber tailing falses with undefs could be reproduced by swapping line 2 above with:

my @args = split /\./, $mystring;
pop @args while (@args and !$args[-1]);
dosumpton(@args);
[download]

Finally, in a very literal sense, your existing branching fails to handle the case where the parent string is defined but major is false - for example, if your version number were 0.11. That will fall right through your block.