Recovering TeX’s short underscore in LaTeX

In $\LaTeX$, writing $\fbox{\verb|the\_long\_underscore|}$ yields $\fbox{\text{the\_long\_underscore}}$, which looks too long when appearing in source code listings. (Here in $\KaTeX$ it looks not that long.) One solution is using the underscore from a monospaced font instead.

Of course, Knuth’s plain $\TeX$ does not have that issue. The underscore is properly sized. Why? Because $\TeX$ does not load a real glyph for $\fbox{\verb|\_|}$. It is defined as $\fbox{\verb|\leavevmode \kern .06em \vbox {\hrule width.3em}|}$, i.e., a horizontal line!

We can recover that good old definition in $\LaTeX$:

$$\begin{array}{l} \verb|\renewcommand{\textunderscore}|\\ \verb| |\verb| {\leavevmode \kern .06em \vbox {\hrule width.3em}}| \end{array}$$

It seems that $\fbox{\verb|\_|}$ is implemented using $\fbox{\verb|\textunderscore|}$, so we do not need to redefine the former. It also seems that $\LaTeX$ does include the good old definition, but somehow overrides it.