Hi,
This is clearly some kind of code that tries to hide whatever nasty things it wants to perform.
So, yes, your server is compromised. Something/someone managed to create that file.
The file is installed in the contact form plugin directory, but there could be more. Search
your whole site for files containing 457563643 . This does not also necessarily mean that
this plugin is vulnerable (although it is quite old and does not follow current DW programming
recommandations). You may use the
bureaucracy plugin instead.
At the very least, remove that infected file (non English language files are not required
by DokuWiki). but of course, you should backup everything, check the various logs, find
the vulnerability, fix your system, reinstall a fresh copy, and restore the data.