(bar.+?qux)|(foo)          # capture anything with 'bar' and 'foo' as 
                           # bookends in $1 OR
                           # all other 'foo' in $2
defined $2 ? '123' : $1    # if $2 exists, replace it with 123
                           # otherwise replace $1 back into 
                           # the string
]ge                        # eval globally