Mac Ports keeps on re-installing Perl, Python, etc Every time I want to install something, mac ports keeps on re-installing Perl, Python, etc. although the same packages are already installed on the system. Is there some setting to avoid this?
What you are seeing there are most probably upgrades for your ports. MacPorts will automatically upgrade all dependencies before installing new ports. Check port outdated to see a list of available upgrades and sudo port upgrade outdated to install them.

It is usually recommended to let MacPorts upgrade dependencies before installing new ports to get the latest fixes. However, if you really need to install a port quickly, you can use sudo port -n install to avoid this behavior and just install the new port only.