The usual suggestion, "use strict; use warnings;". if you d that, you will get warnings, which might lead you to think of something else in this case.

As for the problem, your f() had two different uses in different cases:

1. Being used before sort is called. In this case, it returns 1, and that 1 is then sorted.
2. Being used as call back of sort to determine the order. In your examples, this is not significant, as your list only have one member.

Look at this very simple example, it shows how the call back is used.

use strict;
use warnings;

print sort f (1,2,3);

sub f {1}
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Another one (guess output first):

use strict;
use warnings;

print sort f (1,2,3,4,5,6);

sub f {($b % 2) <=> ($a % 2)}
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