\documentclass[11pt,a4paper,twoside]{report}

\usepackage[cp1250]{inputenc}
\usepackage{czech,a4wide}
\usepackage{fancyvrb}

\begin{document}

\def\WWW{Test.aaa}
\VerbatimInput{\WWW}
\hrulefill
\def\WWW{\_Test.aaa}
\VerbatimInput{\WWW}

\end{document}
