The subs pragma is a red herring. Remove it, and the problem still holds.

Quite so.
If I'm not mistaken, moving the definition of the non_commutative sub to the end of the script *does* create a purpose for loading the 'subs' pragma iff we want to avoid the use of parens - and it would have been better if the scripts I had presented did precisely that:

use warnings;
use subs qw(non_commutative); #necessary

$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";

sub non_commutative {return 17}

__END__
Outputs:
170
17

27
17
[download]

and

use warnings;
use subs qw(non_commutative); #unnecessary

$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";

sub non_commutative {return 17}

__END__
Outputs:
170
170

27
27
[download]

Cheers,
Rob