Why do I see this port as a leave? How to know if I really can get rid of it? Once I've installed an svn, I need it.

$ port echo requested
...
subversion                     @1.8.1_3


As I understand, this port is important for subversion:

$ port echo depof:subversion
...
curl-ca-bundle


Then I take a look on what is important for it:

$ port echo depof:curl-ca-bundle  
perl5


So as a conclusion, svn needs perl5, right?
But:

$ port echo leaves
...
perl5                          @5.12.4_0+perl5_12


How is that possible? Doesn't leaves: installed ports that are unrequested and have no dependents. mean I don't need perl5? So I need it for svn to work or not?
Perl is a build dependency for curl-ca-bundle. Build dependencies are only needed at build time and can be removed afterwards, which makes perl5 a leaf in your case.