documentclass[a4paper,oneside]{memoir}
usepackage{polyglossia}
setdefaultlanguage{english}
usepackage{catchfile}

% ----------------------------------------------------------------------------------------------------------
newcommand*{showme}[1]{The value is #1.}

% ----------------------------------------------------------------------------------------------------------
makeatletter%
newdimenpr@Height%

newcommand{prRaise}[2]{%
  setlength{pr@Height}{f@size pt}%
  raisebox{#1pr@Height}{#2}%
  }%

makeatother%

% ----------------------------------------------------------------------------------------------------------
defcxltxReadA{CatchFileEdef{cxltxR}{/tmp/CXLTXtempout.tex}{}}
defcxltxReadB{CatchFileEdef{cxltxR}{/tmp/CXLTXtempout.tex}{}cxltxR}

% ----------------------------------------------------------------------------------------------------------
begin{document}

showme{foobar}% => ”The value is foobar”.

showme{cxltxReadB}% => ”The value is -0.2 .”

showme{input{/tmp/CXLTXtempout}}% => ”The value is -0.2 .”

showme{immediateinput{/tmp/CXLTXtempout}}% => ”The value is -0.2 .”

AprRaise{-0.2}{B}C% => "ABC" with B lowered

AprRaise{-0.2 }{B}C% => "ABC" with B lowered (trailing space shouldn't be a problem)

% AprRaise{input{/tmp/CXLTXtempout}}{B}C% !!! => Missing number, treated as zero.

% AprRaise{immediateinput{/tmp/CXLTXtempout}}{B}% !!! => Missing number, treated as zero.

% AprRaise{cxltxReadB}{B}C% !!! => Missing number, treated as zero.

cxltxReadA AprRaise{cxltxR}{B}C%

end{document}