Maybe you should have used PATH=/opt/myperl/bin:$PATH so that your perl5.8.9 is found BEFORE the installed perl5.10.

Try this in a new shell (with unchanged PATH):

PATH=$PATH:/opt/myperl/bin perl -v
PATH=/opt/myperl/bin:$PATH perl -v
[download]

If I'm right, you should see the version output of 5.10 and then the version output of 5.8.9