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if ($query_str='') {
    ...
}

if ($query_str == '') {
    ...
}

if ('' == $query_str) {
    ...
}

if ('' = $query_str) {
    ...
...
#   Can't modify constant item in scalar assignment at example.pl line
+ 3,
#   near "$query_str) "
#   Execution of example.pl aborted due to compilation errors.