At first glance it sounds like a great idea. And it is to a certain extent. But there is an existing system in Perl that does that very thing, although only with global variables.

[paul@paul-laptop nqp]$ perl -we '$x = 1'
Name "main::x" used only once: possible typo at -e line 1.
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Speaking from experience, this warning has never helped me. It has done the opposite. It has always been a hinderance. It has forced me to write things similar to the following:

my $value = $SOME_PKG::SOME_VAL
   || $SOME_PKG::SOME_VAL; # warn clean
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Arguably, I should have a function in SOME_PKG that returns the value of $SOME_VAL. But there are many existing modules that don't provide accessors.

Use strict catches all of my typos. A warning of this sort never has.

Still - I could see a use for it, but I think I'd want it as an extra-extra pragma:

use warnings qw(single_use_vars);
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