That gave me

bl123 and barthis123thatqux and barso123quxhim and123som 123
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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;
[download]

gives

bl123 and barthisfoothatqux and barsofooquxhim and123som 123
[download]