I think Ithe most elegant solution is the eval so far. (Thanks for the other suggestions!) But I think I will combine the results of the suggestions, and will come up with a new Class, which will be returned if an undefined or nonexistent hash key has been found. So I will do module like this:

package TableMap::Undef;
use strict;

use overload
  "0+" => sub { return undef },
  "bool" = sub { return undef };

sub new{
  my $a={};
  my $x=shift;
  bltess ($a,ref($x)||$x);
  tie(%$a,"TableMap::Undef::TIE");
  return $a;
};

sub AUTOLOAD { return new TableMap::Undef; };

sub DESTROY {};

package TableMap::Undef::TIE;
use strict;

sub TIEHASH {
  my $a={};
  my $x=shift;
  bless ($a,ref($x)||$x);
  return $x;
};

sub FETCH { return new TableMap::Undef; };
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Every time I need to return an undef I will return "new TableMap::Undef". This object is treated as undef as bool and numeric value, so this will cause right results when we test the value. This problem wuold be solved with a new perl pseudo-variable, we can call it $LAST for example, this is the value of the last similar level tag in the current expression. For example, I want to do things like this:

my $x=$s->{db} && $LAST->{users} && $LAST->{$user_id_1} && $LAST->limi
+ts && $LAST->{$user_id_2};
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What do you think about it? Will it be a good feature request in perl6?