Not sure if that was the original behavior back in 1999 when they were first introduced but newer versions certainly disagree with you:

my %foo;
get_set( 'foo' ) = 42;
print get_set( 'foo' );

sub get_set :lvalue {
    my $key = shift;
    print "key is $key\n";
    my $junk;
    $key eq 'foo' ? $foo{$key} : $junk;
}
__END__
key is foo
key is foo
42
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Where $bad_key is any and all bad keys. This is why one of the desired features would be to determine if it was in assignment or not.