A bug exists in the pecl cli script on freebsd that will not permit you to install any pecl package.

The problem is that the pecl script needs pcre.so but the script calls php with -n ( don't use php.ini ) so the pcre.so extension is not loaded.

A quick fix is to edit /usr/local/bin/pecl and remove "-n" from the last line so it will look like
exec $PHP -C -q $INCARG -d output_buffering=1 -d safe_mode=0 -d register_argc_argv="On" $INCDIR/peclcmd.php "$@"

I found this bug when I wanted to install the svn extension but it seems the bug is known for some time and there is a patch for it : http://www.freebsd.org/cgi/query-pr.cgi?pr=110209