I've done that as well, but if you keep reuse the same shell for a while, switching backwards and forwards, you can run reach the limit of what the shell will allow. And if you compile perl yourself, the paths can be quite long with the default options. I've set up a couple of batch files to call whatever perl I want:
:: perl580.bat @echo off d:\dev\perl\5.8.0\bin\MSWin32-x86-multi-thread\perl5.8.0 %*
> perl561 foo.pl # runs perl5.6.1 > perl580 foo.pl # runs perl5.8.0
I have similar batch files set up for perldoc et al.