Hey,

Just for the fun of it.

#!/usr/local/bin/perl
 
package MyTie;
use Tie::Hash;
@ISA=(Tie::ExtraHash);
 
sub STORE {
   my $self=shift;
   my $key=shift;
   my $value=shift;
   if ( grep /^$key$/, @{$self->[1]} ) {
      warn "key $key not valid";
      return undef;
      }
   return $self->[0]{$key}=$value;
   }
 
1;
 
package main;
use strict;
use Data::Dumper;
 
my $rhash;
tie %$rhash, 'MyTie', [qw(not ok)];
 
$rhash->{b}=2;
$rhash->{not}=2;
 
warn Dumper($rhash);
 
1;
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Of course, the solutions given above are much better, but isn't perl just a lot of fun.......
Of course, the compiler will not complain on this solution, so it is only a half solution.