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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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

# Produces:
#   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.