I'm all for one-liners and golfing, etc, but... Did you know that you can actually include line-feeds when typing in a command-line script like this? You post would work the same if entered as:

perl -e 'for(@ARGV){
 open(I,"<$_");
 s/\.[^\.]*\-[^\-]*$/.MERGED.txt/;
 open(O,">>$_");print O $_ while <I>;close(O);
 close(I)
}' file.abc.part-1.txt file.abc.part-2.txt file.abc.part-3.txt file.de
+f.part-1.txt file.def.part-2.txt
[download]

The line still gets a little long when all those file names need to be filled in too. (Why didn't you use "*.part-*" for that?)

My point is, I've been seeing a lot of this one-liner stuff being offered recently, to people who probably can only scratch their heads and say "That should have been posted in the Obfu wing...".

You'd get more appreciation (and more people would actually be more likely to learn your nifty tricks) if you made it at least a little more legible.