I hope you are not using DOS, that was 16-bit. Presumably you mean cmd.exe. First uninstall existing versions of Perl, using the control panel. Now install your new version.

The PATH environment variable will probably be updated by the installation, you can check this with the path command from cmd.exe (make sure you start cmd.exe after the installation). You might also wish to check the file association with assoc .pl and ftype Perl.

You have to be careful if you have more than one Perl installation on the same machine because these three items may all be different in the registry.