Well, technically the OP does say "surrounded", which could mean the boundaries are switched. Your usage of parens in the example is misleading, because parens are inherently related to internal grouping

But I imagine (not tested) a simple extension of your original regexp could be in order. 

$str =~ s[(bar.+?qux)|(qux.+?bar)|(foo)][defined $3 ? '123' : (defined
+($2) ? $2 : $1)]ge;

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In reply to Re^4: Match a pattern only if it is not within another pattern by Transient
in thread Match a pattern only if it is not within another pattern by punkish

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