Not relevant for this specific case, but there is a difference if the thing being appended is a number:

$ perl -e'use Devel::Peek; my $x = 5; Dump($x) ; my $y .= 5; Dump($y);
+'
SV = IV(0x2fc84) at 0x2fc88
  REFCNT = 1
  FLAGS = (PADMY,IOK,pIOK)
  IV = 5
SV = PV(0x13838) at 0x2fcd8
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x2ac18 "5"\0
  CUR = 1
  LEN = 12
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i.e. .= will stringify on assigment.