Strings mostly act as if they were immutable in Perl 5, so my statement should be taken with care. It's only when you look at the XS or C layer that you see mutable strings: 

use Devel::Peek;

my $x = "foo";

Dump $x;
vec($x, 0, 2) = 3;
Dump $x;

[download]

Output: 

SV = PV(0x1b6a088) at 0x1b8b340
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x1b7a9a0 "foo"\0
  CUR = 3
  LEN = 8
SV = PV(0x1b6a088) at 0x1b8b340
  REFCNT = 2
  FLAGS = (PADMY,POK,pPOK)
  PV = 0x1b7a9a0 "goo"\0
  CUR = 3
  LEN = 8

[download]

You see two different strings at the same memory location, indicating mutability. Though it could of course be a newly allocated copy of the string, while the old one is gone.

Maybe it would have been better if I had talked about hashes being mutable and numbers being immutable.

My main point remains: Perl has some mutable and some immutable data types, and that all languages I know have that distinction too. So a line must be drawn, and in some contexts it might sense to create immutable objects.

Perl 6 - second systems done right

In reply to Re^11: The fallacy of the *requirement* for read-only instance variables. by moritz
in thread The fallacy of the *requirement* for read-only instance variables. by BrowserUk

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