$_ is less special than you think. It wouldn't have worked with a global named $foo either. The eval-solution you gave has already been given (by yours truly).

use vars qw($foo);
{
    $foo = 'foo';
    sub xyzzy {
        print "$foo\n";
    }
}
$foo = 'bar';
xyzzy(); # Prints bar now.
[download]

But:

{
    my $foo = 'foo';
    sub xyzzy {
        print "$foo\n";
    }
}
my $foo = 'bar';
xyzzy(); # Prints the 'foo' that stayed shared.
[download]

Update - s/\$f\b/\$foo/, thanks Kanji.

++ vs lbh qrpbqrq guvf hfvat n ge va Crey :)
Nabgure bar vs lbh qvq fb jvgubhg ernqvat n znahny svefg.
-- vs lbh hfrq OFQ pnrfne ;)
    - Whreq