I just added:

my @dummy=sort byVersion ();
[download]

To the top of my first example and that actually 'solves'
the problem. No error message appeared and the program actually
returns:

./ssrmpilot@IBP-test$ ./sort.s
1.1.1.1 is less than 1.1.1.2

I guess this means that when the compiler finds a certain
subroutine in combination with sort somewhere in the
code, it decides that it must be a 'sort'-function and will
therefore be treated different than a regular subroutine ...

.... I'm still puzzled .... ;-)