my %errors =
  ( $data =~ m#(?:<\%def \.errors>|\G).*?(?:(^\w+):(.*?)$)(?=.*?</\%de
+f>)#sgm );
[download]

It's not immune to matching before or after the tags, though. That gets uglier:

my %errors =
  ( $data =~ m#(?:<\%def \.errors>|(?<=.)\G)(?:(?!</\%def>).)*?(?:(^\w
++):([^\n]*))(?=.*?</\%def>)#sgm );
[download]