For clarification of the question:

I had to read multiple times, because my first instinct was using a Tie::Scalar mechanism and you ruled out Tie::Watch .

But tie has no callback to report changing of the internal representation. Mostly because it's supposed to be transparent.

The only way I see in pure Perl without XS is using the debugger with a complicated watch expression.

This will come with a considerable speed penalty

It might help to know why you need this and which problem you are trying to solve.

Cheers Rolf
(addicted to the Perl Programming Language :)
see Wikisyntax for the Monastery

update

It might be worth checking if $c >120 triggers a tie FETCH, like that you could trace the internal casting and raise an alarm after a change happened.

Update

SO: How are scalars stored 'under the hood' in perl?

In reply to Re: how watch a variable by LanX
in thread how watch a variable by xiaoyafeng

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