This is a simplified high level view of relations between Perl scalar types.

                             ┌───────────────┐
                      ┌───── │ REFERENCE     │ ─────┐
                      │      │ (ROK flag on) │      │
                      │      └───────────────┘      │
      numeric context │                             │ string context
                      │                             │
                      ▼                             ▼
┌─────────────────────────┐                     ┌──────────────────────────────┐
│ NUMBER                  │    string context   │ TEXT STRING                  │
│ encoded internally      │ ──────────────────▶ │ (POK flag on)                │
│ as any of:              │                     │ encoded internally           │
│ * integer (IOK flag on) │   numeric context   │ as one of:                   │
│ * double (NOK flag on)  │ ◀────────────────── │ * iso-8859-1 (UTF8 flag off) │
└─────────────────────────┘                     │ * utf8 (UTF8 flag on)        │
                  │     ▲                       └──────────────────────────────┘
                  │     │                         ▲     │           ▲
                  │     │                         │     │           │
             pack │     │ unpack           decode │     │ encode    │
                  │     │                         │     │           │
                  ▼     │                         │     ▼           │ :encoding
                ┌─────────────────────────────────────────┐         │ PerlIO
                │ BINARY STRING                           │         │ layer
                │ (POK flag on)                           │ ◀───────┘
                │ (UTF8 flag off)                         │
                └─────────────────────────────────────────┘
                        ▲  ▲  ▲
                        │  │  │
                        │  │  │
                        ▼  ▼  ▼
                  ┌─────────────────────────────┐
                  │ OUTSIDE PERL                │
                  │ files, sockets, filenames,  │
                  │ environment, system calls   │
                  └─────────────────────────────┘

(A Perl programmer does not have to know about the internal flags ROK, IOK,
NOK, POK, and UTF8, but if you're interested read perlguts.)


Keep text and binary strings/semantics separated! (Good style anyway!)

If you don't keep them separate, and use a binary string as a text string, it
is assumed to be iso-8859-1 encoded.

If you don't keep them separate, and use a text string as a binary string, one
of the following things happens, with or without warnings:

  1. the internal iso-8859-1 buffer is used (always the case if the internal
     buffer is not utf8 encoded)
  2. the internal utf8 buffer is used
  3. the iso-8859-1 encoded version is used
     3a. characters above U+00FF are utf8 encoded, while the rest is iso
     3b. characters above U+00FF are modulo'ed 256
     3c. characters above U+00FF are dropped
     3d. characters above U+00FF cause an exception to be thrown

If you do keep them separate, and always explicitly convert between the two
types by explicitly decoding and encoding or using the :encoding layer on a
filehandle, you stay in control of what happens and your program will behave
more predictably.
Update: thin lines used, see discussion below.

In reply to How scalars work (about numbers, text strings, and binary strings) by Juerd

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