Re^2: Match a pattern only if it is not within another pattern

[Transient]

That gave me

bl123 and barthis123thatqux and barso123quxhim and123som 123

Update:
However this seems to do what is intended, at least with the test string:

$str =~  s/((?<!bar)\S*)foo((?!\S*qux))/${1}123${2}/gx;

gives

bl123 and barthisfoothatqux and barsofooquxhim and123som 123