Here is one way of approaching the problem using Filter::Simple to remove any my declarations and thereby generate a "used once" warning.

Place the following in a module called WarnUsedOnce.pm.

    package WarnUsedOnce;

    use Filter::Simple sub {
        my $self = shift;

        s/my//g; # Too simple
    };

    1;
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Then use the package in the code you want to test, and run the program with perl -c program.pl:

    use WarnUsedOnce;

    my $var1 = 1;
    my $var2 = 1;
    my @array;

    $var2 = 1; # $var2 used twice

    # Turn the warnings off for the following code
    no WarnUsedOnce;

    my $var3 = 2;

    __END__
    Warnings:

    Name "main::array" used only once: possible typo at test.pl line 5
+.
    Name "main::var1" used only once: possible typo at test.pl line 3.
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Some caveats:

• The my substitution is far too simple, it is only an example..
• You will also get a lot of "Useless use of a variable in void context" warnings with -w.
• Unused variables with the same name but in different scopes won't be detected.
• It probably won't work for pathological cases which in Perl means simple everyday cases. ;-)

So treat this as a proof of concept only.

--
John.