If asked for default imports, File::Find apparently does some magic to make $File::Find::prune known that is not done if you explicitly import only the find procedure.

No, it has nothing to do with import.

Bar.pm:

package Bar;
use warnings;
use strict;

our $x;

1;
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foo.pl:

use warnings;
use strict;
use FindBin;
use lib $FindBin::Bin;

#use Bar; # no warning
#use Bar (); # no warning
#BEGIN { require Bar; } # no warning
require Bar; # warning when var used only once

$Bar::x = 1; # warning with plain 'require'

#$Bar::x = $Bar::x = 1; # no warning
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Edit: Added use Bar (); example.