Or using my newlyfound perlrun knowledge:

When there never are spaces in the end of the 12 characters, and you're sure all lines do contain 12 non-space characters:

perl -i -lpe 's/\s+^//' myfile
[download]

Or the foolproof way:

perl -lne '/^.{1,12}/; printf "%-12s\n",$&' myfile
[download]

I think I just started playing perlgolf...