To be more specific, -W tests to see if a file is writable by the real uid/gid. It would be used like:

if (-W $file) {
   ... do happy stuff ...
}else{
   ... do unhappy stuff ...
}
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Whereas -w is a switch used when running Perl to add a behaviour to the interpreter as it compiles and runs your code. You can see about the file test swicthes by typing:
perldoc -f -X

However, there is a -W switch for Perl 5.6 to turn on all warnings.

Cheers,
KM