rxvt-unicode currently installs Perl extensions in
/usr/lib/$DEB_HOST_MULTIARCH/urxvt/perl. However, the FHS identifies
/usr/share as the preferred tree for architecture-independent data;
Perl scripts should thus be installed in /usr/share.
This is definitely a wishlist item: The FHS says that
“application-specific, architecture-independent directories ... may,
however, be placed in /usr/lib for backwards compatibility, at the
distributor’s discretion.” Eventually, though, it would be nice to get
all the Perl into /usr/share.