The DOS 'copy' command has a -b option to act upon files in 'binary mode,' analogous to Perl's binmode; and depending upon your 'cat' implementation, the fileT and file1 are most likely read through memory in chunks (4K? I think? --something about that being the optimal size for disc transactions...), so that should be a valid way to do this without bring the whole file into RAM. e.g.

[dos-prepend.bat]

  copy /b prepend+original newfile
  if errorlevel 1 goto abort
  copy /y newfile original
  if errorlevel 1 goto abort
  del newfile
  del prepend
abort: rem end

# vs.

[unix-prepend]
#!/bin/sh
  cat prepend original > newfile && (
    mv -f newfile original;
    rm -f prepend )
[download]

Of course, dkubb's subroutine is a much better way to cope with this situation.