Thank you!
The lack of sleep takes its toll... -.-
Yes, with a debug print command I'd have noticed that I changed the order of arguments in the function... *facepalm*
Now it works like as it should, with arguments in the subroutine and its call in the same order, and without the exec.
I guess my error somehow made Perl call sh instead of sending the command to execvp directly, leading to the confusing message.