Here's with s///, just in case your 'a' and 'b' are generic, and could actually consist of longer strings: 
my %replace = ("a" => "b", "b" => "a");
s/(a|b)/$replace{$1}/g;

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In reply to Re: Multiple replacements with the exception of previously made replacements. by JavaFan
in thread Multiple replacements with the exception of previously made replacements. by jthiel

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