\fp_mul:Nn or decode and replace an old command

Question

I have a XeLaTeX document which worked once, today no more, despite that I have installed texlive-full. The document contains a lot of mysterious code which I have grabbed from different sources, and the now improper part resides on the mysterious side.

To be specific, there is a command \varul which probably turns a letter into its variation or whatever, which I can not check because the document does not compile. The definition of the command looks as follows (together with a minimal running example)

\documentclass{article}
\usepackage{fontspec}
\usepackage{lmodern}
\usepackage{soul}
\usepackage{xcolor}


\makeatletter

\ExplSyntaxOn
\cs_new:Npn \white_text:n #1
  {
    \fp_set:Nn \l_tmpa_fp {.01}
    \fp_mul:Nn \l_tmpa_fp {#1}
    \llap{\textcolor{white}{\the\SOUL@syllable}\hspace{\fp_to_decimal:N \l_tmpa_fp em}}
    \llap{\textcolor{white}{\the\SOUL@syllable}\hspace{-\fp_to_decimal:N \l_tmpa_fp em}}
  }
\NewDocumentCommand{\whiten}{ m }
    {
      \int_step_function:nnnN {1}{1}{#1} \white_text:n
    }
\ExplSyntaxOff

\NewDocumentCommand{ \varul }{ D<>{5} O{0.33ex} O{0.1ex} +m } {%
\begingroup
\setul{#2}{#3}%
\def\SOUL@uleverysyllable{%
   \setbox0=\hbox{\the\SOUL@syllable}%
   \ifdim\dp0>\z@
      \SOUL@ulunderline{\phantom{\the\SOUL@syllable}}%
      \whiten{#1}%
      \llap{%
        \the\SOUL@syllable
        \SOUL@setkern\SOUL@charkern
      }%
   \else
       \SOUL@ulunderline{%
         \the\SOUL@syllable
         \SOUL@setkern\SOUL@charkern
       }%
   \fi}%
    \ul{#4}%
\endgroup
}

\makeatother

\begin{document}
    \varul{v}
\end{document}

Searching for ancestors of this scroll yielded a single result https://github.com/dfm/imprs/blob/master/mcmc/dfm_stylez.tex. Interestingly, it exists there in a commented out state. I thus know no more, but this friendly communicate:

! Undefined control sequence.
\white_text:n ...et:Nn \l_tmpa_fp {.01}\fp_mul:Nn 
                                                  \l_tmpa_fp {#1}\llap {\tex...
l.47 ^^I\varul{v}

?

An attempt at a return to the command line with a stochastic combination of ^C and ^D possibly gives some more hints:

? ^C! Interruption.
<to be read again> 
                   \s__fp 
l.47 ^^I\varul{v}

? 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! LaTeX error: "kernel/misused-fp"
! 
! A floating point with value '0.01' was misused.
! 
! See the LaTeX3 documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

l.47 ^^I\varul{v}

? ! Undefined control sequence.
<argument> \textcolor 
                      {white}{\the \SOUL@syllable }\hspace {\fp_to_decimal:N...
l.47 ^^I\varul{v}

? [1] (./test.aux
^C
! Interruption.
l.7397927

?

Could you help me make the document compile with the current XeLaTeX? I guess it is something easy for someone using TeX directly. I would just want to compile the document and have it look more or less like it id.

I am open to any modifications of the \varul macro. Unfortunately, I do not know how it exactly changes/decorates the letter. Might be it was some kind of dotted underline? Can it be guessed from its definition?

Edit: Thanks egreg and TeXnician. So it is now seen that this command makes a white outline around text so that it does not clash with the underscore. I should have added a relevant comment to it. Egreg did not want his answer to be edited so I add an example output, which illustrates the outline:

Answer 1

It's not clear what the purpose of the macro is. However, \fp_mul:Nn does not exist any more.

\documentclass{article}
\usepackage{xparse}
\usepackage{lmodern}
\usepackage{xcolor,soul}

\makeatletter

\ExplSyntaxOn
\cs_new:Npn \white_text:n #1
  {
    \llap{\textcolor{white}{\the\SOUL@syllable}\hspace{\fp_eval:n{0.01*#1} em}}
    \llap{\textcolor{white}{\the\SOUL@syllable}\hspace{-\fp_eval:n{0.01*#1} em}}
  }
\NewDocumentCommand{\whiten}{ m }
    {
      \int_step_function:nnnN {1}{1}{#1} \white_text:n
    }
\ExplSyntaxOff

\NewDocumentCommand{ \varul }{ D<>{5} O{0.33ex} O{0.1ex} +m } {%
\begingroup
\setul{#2}{#3}%
\def\SOUL@uleverysyllable{%
   \setbox0=\hbox{\the\SOUL@syllable}%
   \ifdim\dp0>\z@
      \SOUL@ulunderline{\phantom{\the\SOUL@syllable}}%
      \whiten{#1}%
      \llap{%
        \the\SOUL@syllable
        \SOUL@setkern\SOUL@charkern
      }%
   \else
       \SOUL@ulunderline{%
         \the\SOUL@syllable
         \SOUL@setkern\SOUL@charkern
       }%
   \fi}%
    \ul{#4}%
\endgroup
}

\makeatother

\begin{document}

X\varul{abc def}X

X\varul<20>[1ex][1ex]{abc def}X

\end{document}