It looks like you want everything after "-f" up to "makefile", how about the following:

use strict;

while(<DATA>){
  print m|-f\s+(\S+makefile)|i;
}
__DATA__
make -r -f ..\../tools/cell/makefile cell.lib
[download]

Depending on how you're testing your regex, your problem could be whether you're escaping the "\" in your string.