Well, if what you want is to do the transliteration only inside the TIER element, you can use flip/flop.

if (/\<TIER/ .. /\/TIER\>/) {
    $input =~ $your_substitution_regex;
}
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That's a quick and dirty solution but it's cheap so you may just be happy with it if you don't want to seriously parse the XML.

See the perlop man page for more info about this operator.