It means that the library (Common.so) is using that symbol, but the symbol is undeclared ("U") within the library itself, i.e. it's an external reference that needs to be satisfied by some other library.

The symbol is most likely expected to be defined in the shared library libxml2.so that Common.so is presumably linked against — that library contains the actual implementation of most of the functionality; Common.so is just the Perl/C wrapping code...  In other words, you could try to run "nm -D" on libxml2 to see if the symbol is defined there  (check the build output for where the lib was found). The symbol should not be listed as "U" there, but rather as something like "T" ("text" section, aka code), or "D" ("data" section).

Update: just checked on my box with v2.6.32 and an older box with v2.6.31, and the symbol is defined (in both cases):

$ nm -D /usr/lib/libxml2.so.2.6.32 | grep xmlFree\$
000000000035a6c0 D xmlFree
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