others have addressed your problem, however, the reason for the warning only showing up at the last of the elsif's can be shortened to this:

perl -wle'-2;
-1;
0;
1;
2;'
Useless use of a constant in void context at -e line 1.
Useless use of a constant in void context at -e line 2.
Useless use of a constant in void context at -e line 5.
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1 or 0 in void context aren't treated as useless. that's why you can write 1; at the end of a module, and, uhm, i can't think of an example for the zero at the moment...