Your code as posted

$ perl -MO=Deparse,-p junk
use subs ('non_commutative');
sub non_commutative {
    use warnings;
    return(17);
}
use warnings;
($x = (10 * non_commutative()));
($y = non_commutative(*10));
print("$x\n$y\n\n");
($x = (10 + non_commutative()));
($y = non_commutative(10));
print("$x\n$y\n");
junk syntax OK
[download]

If you want a function that doesn't take arguments, use a prototype of (), as in sub non_commutative() {return 17}

 $ perl -MO=Deparse,-p junk
sub non_commutative () {
    use warnings;
    return(17);
}
use warnings;
($x = (10 * non_commutative));
($y = (non_commutative * 10));
print("$x\n$y\n\n");
($x = (10 + non_commutative));
($y = (non_commutative + 10));
print("$x\n$y\n");
junk syntax OK
[download]

or use constant non_commutative => 17;

$ perl -MO=Deparse,-p junk
use constant ('non_commutative', 17);
use warnings;
($x = 170);
($y = 170);
print("$x\n$y\n\n");
($x = 27);
($y = 27);
print("$x\n$y\n");
junk syntax OK
[download]