How to pass an array to drupal7 form select as value of #options?
Have a look at the Form API docs for select lists. #options is the correct property to set the options, and #default_value is used to set a default value. 

Using #value means the user input will be ignored, and whatever you've set as #value will wind up being the submitted value. Since you're declaring a non-multiple select list, that value needs to be an integer (not an array) anyway.

Try something like this:

$region_value = array(0 =&gt; 'US', 1 =&gt; 'EU');
$form['region_options'] = array(
  '#type' =&gt; 'select',
  '#prefix' =&gt; 'Region',
  '#options' =&gt; $region_value,
  '#default_value' =&gt; 0, // e.g. Set US as default selected value (appropriate array key from $region_value)
);