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