|Welcome to the Monastery|
Thank you; indeed, Perl has no guilt.
As I was trying to track down a slightly more complicated issue, I just was on the wrong track for a while.
Looks like _my_ real problem originates from something I now believe to be an en_US.UTF-8 locale/glibc bug or problem which I can reproduce at system level.
Given a simple file having two records with two TAB separated fields each, like
a_2 2 a2 1a command like 'sort -k 1 | cut -f 1 ' on that file displays
a2 a_2(as if the field delimiter wouldn't work and the second field determines the ordering), whereas on the same file _without_ the second field the sort order is reversed.
This looks like a bug to me, tough I'm not sure yet and don't really know so far whom I should or could report this.