Presuming some variant of *NIX and a shell that groks {00..49} notation (zsh, and I believe bash):

mkdir out
for i in infiles{01..49} ; do
  ( cd out ; split -l 1024 -a 5 ../$i ${i:s,infiles,,} )
done
for i in $(perl -le 'print for "aaaaa".."aaazz"') ; do
  cat out/{01..49}$i > segment_$i
done
rm -rf ./out
for i in segment_* ; do
  processing $i
done
[download]

Modifying to automagically determine the maximum split partition name left as an exercise for the reader.