Why does the pragma order matter when enabling experimental features?

Question

Is it the defined behavior that the order of features and use version matters?

use feature 'signatures';
use v5.026;

vs

use v5.026;
use feature 'signatures';

The top will produce an error,

Global symbol requires explicit package name

on

use feature 'signatures';                                                                                        
use v5.026;                                                                                                      

sub foo ($opt1, $opt2 = undef) {                                                                                 
  say $opt1 if $opt2;                                                                                            
}

Answer 1

The use v5.026 declaration implicitly enables the feature bundle for that version, in this case it would be equivalent to no feature ':all'; use feature ':5.26'. This overrides the features you had enabled/disabled already.