perl -e '$self->{a$_};print keys %$self, $/;'
--------
Can't call method "a" on an undefined value at -e line 1.

##</code><code>##

package Foo;

sub new { my $c = shift; bless \$c, $c }
sub bar { 'bar' }

package main;

$foo = Foo->new;
$x{bar$foo} = 1;
print "$_ => $x{$_}\n" for keys %x;

--------

bar => 1