If you're running ActiveState Perl, doesn't it take care of this for you? If you go to a dos window and type path, see if Perl is in your path already.
If not:
Using notepad, open c:\autoexex.bat.
Once you have that open, look for a line with SET PATH in it. ( Mine looks like SET PATH=C:\Perl\bin\;%PATH%
After the semicolon, if it doesn't exist, put in the path to your perl