I'm no Windows person, but I know that Perl's internal processing of system arguments is rather complex... so maybe the difference of this example here (which works) to the other case with the quoted argument has the effect that the latter is executed via cmd.exe, while the former executes files/pdftotext.exe directly (in which case the forward slash wouldn't be a problem).  Just a guess, though, as at the moment I don't have a Windows to try.