I read a file in my GuiXT script with 
Readfile &V[Filename] M1
This works well, but leading space characters are lost in M1. They are important for my application. Is there a workaround?

Yes, you can read the whole file into a text variable first. Example:

  // copy whole file into text

CopyText fromFile="C:\temp\tkl.txt" toText="tkl"

  // line index

  Set V[i] 1

  label read_line

CopyText fromText="tkl" toString="rec" line="&V[i]"

if Q[ok]

  // do something with variable V[rec]

  // increase line count

  Set V[i] &V[i] + 1

  goto read_line