Simple one liner problem.

[SAWMaster]

I can't seem to use variables like this....why not?
Is there a simple way to do this?
Without having to manually do it with an array traversing check/change function?

$workingcopy =~ tr/$lettertochange/$lettertochangeto/g;

I also tried

$workingcopy =~ s/$lettertochange/$lettertochangeto/g;


Thanks

-Dave