An interesting question.

Firstly, let's discount exporting package variables. It's easy enough to do, and while they can't be lexically scoped, you can provide a dynamic-scoped value (similar to the effects of local). You'd just set up the variable in your import method, and use B::Hooks::EndOfScope to restore the old value at the end of the scope that imported you...

use strict;
use warnings;

BEGIN {
    package MyVars;
    use Exporter::Shiny;
    use B::Hooks::EndOfScope;
    
    sub import {
        no strict "refs";
        my $caller = caller;
        my $old_value = ${"$caller\::pi"};
        *{"$caller\::pi"} = do { my $new_value = 3.14159; \$new_value 
+};
        on_scope_end {
            *{"$caller\::pi"} = \$old_value;
        }
    }
};

use Data::Dumper;

{
    use MyVars;
    BEGIN { print Dumper($pi) }
}

BEGIN { print Dumper($pi) }
[download]

However, note that the print statements in the importing package need to happen at compile time; this is because our dynamic definition of the variable only happens at compile time; by run time it's gone.

So that's a useless idea. On to lexical variables.

The tricky part is actually declaring the lexical variable in the caller's scope so that when they use the variable, it doesn't result in a compile-time error. This requires using parser hooks. B::Hooks::Parser is pretty reliable, and works on Perl 5.8+.

use strict;
use warnings;

BEGIN {
    package MyVars;
    use Exporter::Shiny;
    use B::Hooks::Parser;
    
    our $pi = 3.14195;
    sub import {
        B::Hooks::Parser::inject(sprintf('my $pi; BEGIN { $pi = $%s::p
+i };', __PACKAGE__));
    }
};

use Data::Dumper;

my $pi = 3;

{
    use MyVars;
    print Dumper($pi);
}

print Dumper($pi);
[download]

With a little extra work (and Data::Alias), you could make the "exported" $pi into an alias for the $MyVars::pi package variable. Or you could make it read-only. Or a tied variable, or an overloaded object, or whatever.

Update: I've packaged up this solution as Exporter::LexicalVars. I'll probably release a stable version in a few days.

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