The line numbers in your error messages do not agree with the line numbers in the source code which suggests either copy errors or editing has taken place. So copy it accurately and run it without editing it.

The code runs as expected when I copy and paste the code from the node above.

Note that if I remove the my from the %frequency_table declaration and add a my to the $key_string .= assignment I get the same pattern of errors (with slightly different line numbers) that you report.