You can do the same thing but add Regexp::Common for arbitrary balanced delimiters;

use Regexp::Common;

my $orig = my $str = 'foo bar foo bar foo qux foo qux foo';

$str =~ s{ ( $RE{balanced}{-begin => "bar"}{-end => "qux"} ) | (foo) }
         { defined $2 ? 123 : $1  }xge;

print "$orig\n";
print "$str\n";
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Result:

foo bar foo bar foo qux foo qux foo
123 bar foo bar foo qux foo qux 123
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