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.def.part-1.txt file.def.part-2.txt