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.

In reply to Re: Internal error from Perl by graff
in thread Internal error from Perl by John M. Dlugosz

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