Sadly, I don't think my version of perl supports Hash::Util

You're using older than 5.8.0? Isn't that at least 5 years old?

I'm not sure, and a quick scan of perldelta didn't tell me, when Internals first appeared, but it can do the same job with a minor additional amount of work. Once you have built your hash, you have to set the values readonly as well as the hash itself. Hardly onerous though:

#! perl -slw
use strict;
use Internals qw[ SetReadOnly ];

my %set = map{ $_ => 1 } qw[ age old years ];
SetReadOnly( \$_ ) for values %set; ## Freeze values
SetReadOnly \%set;                  ## and keys.

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

eval{ $set{ age } = 123 } or print $@;
eval{ $set{ fred } = 1 } or print $@;

__END__
C:\test>junk3
age exists
sex  does not exist
old exists
young  does not exist
years exists
minutes  does not exist
Modification of a read-only value attempted at C:\test\junk3.pl line 1
+7.

Attempt to access disallowed key 'fred' in a restricted hash at C:\tes
+t\junk3.pl line 18.
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