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my %reasons;
while (my($city, $code_info) = each %$switch_hash) {
    my ($code, $date_info) = %$code_info;
...
        $reasons{"$city:$date"} = $2;
    }
}

my $switch_hash = {
    'Washington:uslecwas5e1' => {
        '01-AUG-2002' => ''
...
        '01-AUG-2002' => ''
    },
# ...

my $switch_hash = {
    'Washington' => {
        CODE => 'uslecwas5e1',
        '01-AUG-2002' => ''
    },
# ...

my %reasons;
while (my($city, $info) = each %$switch_hash) {
    while(my ($date, $reason) = each %$info) {
...
        $reasons{"$city:$date"} = $2;
    }
}