chop is destructive, substr isn't.

chop _removes_ the last character from $meta_config{login_cookie_expiration} and returns it (setting $time_frame). substr just returns the last character, without modifying the $meta_config entry.

Obviously, in this case, changing the hash entry is bad. It would get "badder" if you called the routine several times, as you nibbled away at the string character by character.
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Mike