Moose has a special syntax for this, if you don't use it, you are not really using Moose. In your case you could say e.g. (I left use strict / warnings out for brevity):

test.pl

use Person;
my $p = Person->new(age => 10);
print "Person has age ", $p->age();
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Person.pm

package Person;

use Moose;
use Types;

has 'age' => (is => 'rw',
              isa => 'AdmissibleAge');

1;
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Types.pm

package Types;

use Moose::Util::TypeConstraints;

subtype 'AdmissibleAge'
  => as 'Num'
  => where {$_ >= 0 and $_ <= 99};
  
1;
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I like to separate the type constraints in a separate module Types.pm. What I just told you is no news, of course, you can find very good introductions on the Moose homepage, the first aricles I read and which should give you a good start are the ones cited there by Randal L. Schwartz:

http://www.stonehenge.com/merlyn/LinuxMag/col94.html

and

http://www.stonehenge.com/merlyn/LinuxMag/col95.html