documentclass{article}
usepackage{filecontentsdef}
usepackage[enable-debug]{expl3}
usepackage{xparse}
ExplSyntaxOn
debug_on:n{check-declarations,deprecation,check-expressions}
% myscantokens
NewDocumentCommand{myscantokens}{+m}
 {
  group_begin:
  tl_set:Nn l_tmpa_tl {#1}
  tl_put_right:Nx l_tmpb_tl %
    {
     tex_newlinechar:D = 13
     tex_everyeof:D = { exp_not:N }
     exp_not:N scantokens exp_after:wN { l_tmpa_tl }
     tex_newlinechar:D = 10 scan_stop:
    }
  l_tmpb_tl
  group_end:
 }

% mytlrescan
tl_new:N l_tmpc_tl
cs_generate_variant:Nn tl_set_rescan:Nnn { NnV }
NewDocumentCommand{mytlrescan}{+m}
 {
  group_begin:
  tl_set:Nn l_tmpa_tl {#1}
  tl_set_rescan:NnV l_tmpb_tl { } l_tmpa_tl
  tl_put_right:Nx l_tmpc_tl %
    {
     tex_newlinechar:D = 13
     tex_everyeof:D = { exp_not:N }
     l_tmpb_tl
     tex_newlinechar:D = 10 scan_stop:
    }
    l_tmpc_tl
  group_end:
 }
ExplSyntaxOff
pagestyle{empty} % only for example
begin{document}
section{Default texttt{textbackslash filecontentsexec}}

begin{filecontentsdef*}{jobname.tmp}{macroA}
Test environment with verb+filecontentsdef*+. verb+macroA+ its global.par
begin{verbatim}
      A verbatim
end{verbatim}
end{filecontentsdef*}
filecontentsexecmacroA

section{Using texttt{textbackslash scantokens}}
myscantokens{macroA}

section{Using texttt{textbackslash tl_rescan:nn}}
mytlrescan{macroA}
end{document}