fixed.zsh

#!/usr/bin/env zsh

set -e

__handle_flag() {
    local flagname="--namespace="
    local flagval="kube-system"

    flaghash[${flagname}]=${flagval}

    echo "flaghash[${flagname}]=${flaghash[${flagname}]}"
}

__handle_word() {
    __handle_flag
}

__start_kubectl() {
    builtin declare -A flaghash

    __handle_word
}

__start_kubectl

#
# $ zsh fixed.zsh
# flaghash[--namespace=]=kube-system
#

# __start_kubectl {
#
#     builtin declare -A flaghash
#
#     __handle_word {
#
#         __handle_flag {
#
#             # Being able to access flaghash declared in __start_kubectl from here
#             flaghash[${flagname}]=${flagval}
#
#         }
#
#     }
# }

reproduction.zsh

#!/usr/bin/env zsh

set -e

__kubectl_declare() {
    builtin declare "$@"
}

__handle_flag() {
    local flagname="--namespace="
    local flagval="kube-system"

    flaghash[${flagname}]=${flagval}

    echo "flaghash[${flagname}]=${flaghash[${flagname}]}"
}

__handle_word() {
    __handle_flag
}

__start_kubectl() {
    __kubectl_declare -A flaghash

    __handle_word
}

__start_kubectl

#
# $ zsh reproduction.zsh
# __handle_flag:4: bad math expression: operand expected at end of string
#

# __start_kubectl {
#
#     __kubectl_declare {
#
#         builtin declare -A flaghash
#
#     }
#
#     __handle_word {
#
#         __handle_flag {
#
#             # Not being able to access flaghash declared in __kubectl_declare from here
#             flaghash[${flagname}]=${flagval}
#
#         }
#
#     }
# }