First of all, this ...

• sub ten {10}

is not a constant.

For constant folding (optimization at compile time) to happen, you need a a prototype of () , to make sure the result doesn't depend on input.

• sub ten() {10}

Otherwise arguments are allowed, which is biting you here!

The following demos with B::Deparse will demonstrate what is happening

C:\tmp>perl -MO=Deparse,-p -e"sub ten {10}; print ten() - 1"
sub ten {
    10;
}
print((ten() - 1));                                              # <--
+- call no args
-e syntax OK

C:\tmp>perl -MO=Deparse,-p -e"sub ten {10}; print ten - 1"
sub ten {
    10;
}
print(ten((-1)));                                                # <--
+- call with args
-e syntax OK

C:\tmp>perl -MO=Deparse,-p -e"sub ten() {10}; print ten - 1"
sub ten () {
    10;
}
print(9);                                                        # <--
+- constant folding
-e syntax OK

C:\tmp>perl -MO=Deparse,-p -e"sub ten {10}; 9 .. ten - 1"
sub ten {
    10;
}
(9 .. ten((-1)));                                                # <--
+- not what you wanted
-e syntax OK

C:\tmp>
[download]

updates

s/code folding/constant folding/g

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