Does X-Perl even support the arena enemy units? I don't use it, but I didn't see any mention of "arena" on
its download page.
Assuming that it does not include arena frames, you should be able to use the ones built into the default UI (enable them under Interface Options > Game > Unit Frames) or use any other arena frame mod.
If you like Gladius, do you have the
patch 5.2 fan update or the really outdated official version? If you don't have the linked update version, go get it. If you're still getting errors with it, post the errors -- preferrably on the addon's download page so the uploader will see them and can fix them.
If you're okay with the basic appearance and functionality of the Blizzard arena frames,
sArena will let you move and resize them. If you're open to trying unit frame addons other than X-Perl,
Shadowed Unit Frames and
Stuf both support arena enemies, and have lots of configuration options. There are also
oUF layouts that support arena frames, including
mine, though most oUF layouts don't have many (or any) in-game options, so you generally just pick one you like and use it as-is.