if you don't have perl installed on a system, slony won't build:

./configure --prefix=/usr/local
  .. finishes ok ..

gmake ends with this:

gmake[2]: Leaving directory `/var/tmp/n/lorax1/usr/ports/databases/ 
slony1/work/slony1-1.1.0/src/ducttape'
gmake[1]: Leaving directory `/var/tmp/n/lorax1/usr/ports/databases/ 
slony1/work/slony1-1.1.0/src'
gmake[1]: Entering directory `/var/tmp/n/lorax1/usr/ports/databases/ 
slony1/work/slony1-1.1.0/tools'
for subdir in altperl ; do \
           gmake -C $subdir all ; \
         done
gmake[2]: Entering directory `/var/tmp/n/lorax1/usr/ports/databases/ 
slony1/work/slony1-1.1.0/tools/altperl'
The altperl tools require that Perl be installed.
exit 1;
gmake[2]: *** [all] Error 1
gmake[2]: Leaving directory `/var/tmp/n/lorax1/usr/ports/databases/ 
slony1/work/slony1-1.1.0/tools/altperl'
gmake[1]: *** [all] Error 2
gmake[1]: Leaving directory `/var/tmp/n/lorax1/usr/ports/databases/ 
slony1/work/slony1-1.1.0/tools'
gmake: *** [all] Error 2

if you do have perl, it build fine and as expected won't install the  
altperl tools. It seems that the altperl Makefile needs to check  
whether they were requested before throwing an error for  missing  
perl, as they do when installing.