Here is a small working example. Have a look at the Hash of Hashes (HOH) section of the perldsc manpage for further details:

   #!/usr/bin/perl -wl

    use strict;

    # Map the old client names to the new names
    my %map     = (ABCDEF => 'ABCD', GHIJKJ => 'GHIJ');

    # Store the client data in a hash of hash refs
    my %clients = (
                      ABCDEF => {name => 'Sam', age => 30},
                      GHIJKJ => {name => 'Dan', age => 40},
                  );

    # Link the old clients to the data
    foreach my $key (keys %clients) {
        $clients{$map{$key}} = $clients{$key};
    }


    # Print the data, modify it and print it again
    print ${$clients{ABCDEF}}{name}, " is ", ${$clients{ABCDEF}}{age};
    print ${$clients{ABCD  }}{name}, " is ", ${$clients{ABCD  }}{age};

    ${$clients{ABCD}}{age} = 35;

    print ${$clients{ABCDEF}}{name}, " is ", ${$clients{ABCDEF}}{age};
    print ${$clients{ABCD  }}{name}, " is ", ${$clients{ABCD  }}{age};

    __END__
    Prints:

    Sam is 30
    Sam is 30
    Sam is 35
    Sam is 35
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--
John.