In when you are using strict you have to declare your varibles with the mykey word. ie my $foo = "a bar"; then when you use it later on you can just say $foo = "two bars";.
The warning you are getting appears when the variables have not been declared in this way. This is helpful beacuse if you mis-spell a variable then you will be warned of the fact. This has saved me about 250,000 times from stupid typos and the like. There are other reasons too I am sure, but this is why I use it.

Hope this helps.