Thanks for all the replies to my "detecting an undefined ..". I agree with the last reply, that I meant "declared" rather than "defined". It's not clear to me why what I'm trying to do should be so undisciplined (ill advised?). I often put together pieces of code from various places. So it's handy to check whether some variable already exists in another piece. In the example I gave, the code would be in a function being called from some other code, written at a different time. If I had then used a variable called $scale, it would be handy to know whether that variable existed in the calling code or whether I now had to declare and define it anew. If it existed before, I would want to use the existing value. If not, I would define it as 
my $scale = 1;

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That doesn't seem so undisciplined to me.

In reply to Re^3: detecting an undefined variable by LloydRice
in thread detecting an undefined variable by LloydRice

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