I think it works :)

$ perl  import-into-warnings-notToolkit.pl
f is not init but no warning
Argument "A" isn't numeric in addition (+) at import-into-warnings-not
+Toolkit.pl line 19.
f is not a number and you're warned 0
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So whatever the deal is with experimental::smartmatch it might be a bug :)

Here are all warnings forced for comparison

$ perl  -W import-into-warnings-notToolkit.pl
Use of uninitialized value $f in concatenation (.) or string at import
+-into-warnings-notToolkit.pl line 17.
f is not init but no warning
Argument "A" isn't numeric in addition (+) at import-into-warnings-not
+Toolkit.pl line 19.
f is not a number and you're warned 0
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the code

#!/usr/bin/perl --
BEGIN {
    package Fake;
    $INC{'Fake.pm'}=__FILE__;
    use Import::Into;
    sub import {
        my $target = caller;
        'strict'->import::into($target);
        'warnings'->import::into($target);
        'warnings'->unimport::out_of($target, 'uninitialized');
    }
}

package Snake;
use Fake;
my $f ;
print "f is not init but no warning $f \n";
$f = 'A';
print "f is not a number @{[  0 +  $f ]}\n";


__END__
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