#! perl -slw
use strict;
use Hash::Util qw[ lock_hash ];

my %set = map{ $_ => 1 } qw[ age old years ];
lock_hash %set;

for my $unknown ( qw[ age sex old young years minutes] ) {
    print "$unknown ",
        exists $set{ $unknown }
            ? 'exists'
            : ' does not exist';
}

$set{ fred } = 1;

__END__
c:\test>junk3
age exists
sex  does not exist
old exists
young  does not exist
years exists
minutes  does not exist
Attempt to access disallowed key 'fred' in a restricted hash at c:\test\junk3.pl line 15.