How to TeX an arrow with vertical stroke?

Here's a possible way

\newcommand\pto{\mathrel{\ooalign{\hfil$\mapstochar$\hfil\cr$\to$\cr}}}

Note that the definition of oz.sty is equivalent to

\newcommand\pfun{\mathrel{\ooalign{\hfil$\mapstochar\mkern5mu$\hfil\cr$\to$\cr}}}

so it differs from the former only by a slight shift to the left of the bar.

---

A complete and better implementation using also the shift in oz.sty; this one works also in subscripts and superscripts.

\documentclass{article}

\makeatletter
\newcommand{\pto}{}% just for safety
\newcommand{\pgets}{}% just for safety

\DeclareRobustCommand{\pto}{\mathrel{\mathpalette\p@to@gets\to}}
\DeclareRobustCommand{\pgets}{\mathrel{\mathpalette\p@to@gets\gets}}

\newcommand{\p@to@gets}[2]{%
  \ooalign{\hidewidth$\m@th#1\mapstochar\mkern5mu$\hidewidth\cr$\m@th#1\to$\cr}%
}
\makeatother

\begin{document}

$A\pto B\pgets C$

$\scriptstyle A\pto B\pgets C$

\end{document}

You may want to use \pfun in package oz as listed in the overview here.

With unicode-math package (requires XeTeX or LuaTeX) and a proper Unicode math font, you can access that Unicode arrow (and a gazillion of other Unicode math symbols):

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{XITS Math}

\begin{document}
$\nvrightarrow$
\end{document}