use Data::Dumper;
$Data::Dumper::Terse=1;
$Data::Dumper::Useqq=1;
$Data::Dumper::Indent=0;
for my $x (0, "0", "", undef, "true", 1) {
   my $y=$x;
   $y||='?';
   print Dumper($x), " -> ", Dumper($y), "\n";
}
__END__
0 -> "?"
0 -> "?"
"" -> "?"
undef -> "?"
"true" -> "true"
1 -> 1