Here is one way:

s/(^.*<)\:([a-z]+)>/ "$1" . ( exists $inst{ $2 } ? $inst{ $2 } : $inst
+{ default } ) /e;
[download]

or

s/(^.*<)\:([a-z]+)>/ "$1" . ( $inst{ $2 } || $inst{ default } ) /e;
[download]