That looks complicated (though it isn't really).
I'd probably use a temporary:

for my $c ( keys %h ) {
    my @attr = keys %{$h{$c}};
    if ( @attr == 1 and $h{$c}{$attr[0]} == 1 ) {
        print "$c has a $attr[0]\n";
    }
. . .
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jdporter
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