Untested:

s{^(OBX\|[^|]*\|)([^^]*)\^}
 {my ($pref, $str) = ($1, $2);
  $str =~ s{([A-Z])}{"U$1"}g;
  $str =~ s{([a-z])}{"L$1"}g;
  "$pref$str"}eg;
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It'll be really awkward doing this with a single s///.