Are Perl strings immutable?

Question

What's happening behind the scenes when I do a concatenation on a string?

my $short = 'short';
$short .= 'cake';

Is Perl effectively creating a new string, then assigning it the correct variable reference, or are Perl strings always mutable by nature?

The motivation for this question came from a discussion I had with a colleague, who said that scripting languages can utilize immutable strings.

Answer 1

Perl strings are mutable. Perl automatically creates new buffers, if required.

use Devel::Peek;
my $short = 'short';

Dump($short);
Dump($short .= 'cake');
Dump($short = "");

SV = PV(0x28403038) at 0x284766f4
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x28459078 "short"\0
  CUR = 5
  LEN = 8
SV = PV(0x28403038) at 0x284766f4
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x28458120 "shortcake"\0
  CUR = 9
  LEN = 12
SV = PV(0x28403038) at 0x284766f4
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x28458120 ""\0
  CUR = 0
  LEN = 12

Note that no new buffer is allocated in the third case.

Answer 2

Perl strings are definitely mutable. Each will store an allocated buffer size in addition to the used length and beginning offset, and the buffer will be expanded as needed. (The beginning offset is useful to allow consumptive operations like s/^abc// to not have to move the actual data.)