I use the memoir
class, which can typeset poetry with the verse
environment:
\documentclass{memoir}
\begin{document}
\linenumberfrequency{1}
\verselinenumbersleft
\settowidth{\versewidth}{In the cavern}
\begin{verse}[\versewidth]
In the cavern\\
In the canyon\\!
\end{verse}
\end{document}
Which renders:

Memoir
's \settowidth{\versewidth}{In the cavern}
command and the [\versewidth]
argument amount to the \centerpoem{In the cavern}
command of the poem
package. (It might appear that you need more instructions with the memoir
class to attain the same results as with the poem
package; however, the difference mainly concerns the defaults: default line numbering frequency, line numbering position, etc.)
(See the memoir documentation, Ch. 14, for detailed instructions on how to typeset and tweak poetry text.)
In any event, better not use memoir
with the poetry
package.