use Devel::Peek;

my $test = "\x{2345}\x{3456}";

Dump $test;

$test =~ tr/\x{2345}\x{3456}/XY/;

Dump $test;

__END__
SV = PV(0x768bc8) at 0x7907d8
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK,UTF8)
  PV = 0x782630 "\342\215\205\343\221\226"\0 [UTF8 "\x{2345}\x{3456}"]
  CUR = 6
  LEN = 8
SV = PV(0x768bc8) at 0x7907d8
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK,UTF8)
  PV = 0x782630 "XY"\0 [UTF8 "XY"]
  CUR = 2
  LEN = 8