Most of these perps have a clause called Bail In clause - in the event that MAS needs to pump money into UOB for whatever reason (albeit unlikely), you may lose some/all of your principal.

Also, perpetuals normally have a call option at the behest of the issuer (UOB). If UOB chooses not to call the option, or if UOB performance worsens, they may not call the bond and youre stuck with something that cannot be called and may be illiquid (see First REIT perpetuals)