my %test_values_of = (
    '0'        => '1',
    '1'        => '1',
    '4349678L' => '9',
    '4347946L' => '9',
    '791517N'  => '9',
    ''         => '9',
    '791517N'  => ''
);
my $n = 0;
while ( my ( $key, $value ) = each(%test_values_of) ) {
    $n++;
    print $n. '.' . '#' x 20, "\n";

    for $op ( '&', '&&', 'and' ) {
        test_op( $key, $value, $n, $op );
    }
}

sub test_op {
    my ( $key, $value, $n, $op ) = @_;
    print "# [$key] $op [$value] ";
    my $rez = eval( '$key ' . $op . ' $value' );
    if ($rez) {
        print 'done';
    }
    else {
        print 'no';
    }
    print '[' . $rez . "]\n";
}

1.####################
# [] & [9] no[]
# [] && [9] no[]
# [] and [9] no[]
2.####################
# [4347946L] & [9] no[0]
# [4347946L] && [9] done[9]
# [4347946L] and [9] done[9]
3.####################
# [1] & [1] done[1]
# [1] && [1] done[1]
# [1] and [1] done[1]
4.####################
# [791517N] & [] no[]
# [791517N] && [] no[]
# [791517N] and [] no[]
5.####################
# [0] & [1] no[0]
# [0] && [1] no[0]
# [0] and [1] no[0]
6.####################
# [4349678L] & [9] no[0]
# [4349678L] && [9] done[9]
# [4349678L] and [9] done[9]