The problem also goes away if you don't use Foo:

use utf8;
use charnames ':full';

# don't use Foo;

print "Hello world \N{EURO SIGN} \n";

# no error message
[download]

Meanwhile, when using Foo, and using charnames in both main and Foo, you do get the message, but it is just a warning, and the "\N{...}" construct really does work in both name spaces:

use utf8;
use charnames ':full';

use Foo;

print "Hello world \N{EURO SIGN} $Foo::LN \n";

# "unreferenced scalar" message shows up, but so do both 
# intended characters (assuming you have the right font...)
[download]

Anyway, I'd agree that this merits a bug report.