You would benefit from prepending

use strict;
use warnings;
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to all your Perl code from now on. Always.

Your code would probably work if you'd change it into something like:

use strict;
use warnings;

# ...

my %mysql = ();
while (my ($keyword, $value) = $sth->fetchrow_array()) {
    $mysql{$keyword} = $value;
}
print $mysql{"back_log"};
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