My immediate first guess would be:

$ sudo updatedb
$ locate font.ttf
[download]

...and then use that full pathname to the font in your code. Failing that I would look into Imager::Font's code and search for the relevant open() for your file...you must be missing something. Perhaps it's an include path in xft, but this isn't a linux forum.

Quick answer, but if it works, it works. If it doesn't, move onto the next :-)

Merry Christmas,
~texasperl