grep (and for) provide lvalue context for their args, since the expectation is that when aliased to $_, the $_ variable (and thus the arg) could be modified by the code in the code block. This lvalue context is what causes the autovivification action to be compiled in. The 'x' element of %$h is created at run-time when the arg list for grep is being assembled, but before grep itself is invoked (i.e. before the code block is called for the first time).

However, function call arguments which are hash subscripts are special-cased. Although function args are technically always in lvalue context (e.g. the sub about to be called might do sub f { $_[0]++ }, who knows!), perl tries to avoid auto-vivification by deferring hash lookup. Instead, if you do foo($h{$k}) then rather than looking up the hash value and passing it as an arg, perl creates a special proxy object (PVLV) which holds pointers to $h and the key and passes that instead. For example: 

$ perl -MDevel::Peek -e'sub f { Dump $_[0] } f($h{akey})'
SV = PVLV(0xd28a78) at 0xcb2658
  REFCNT = 1
  FLAGS = (GMG,SMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0xce9828
    MG_VIRTUAL = &PL_vtbl_defelem
    MG_TYPE = PERL_MAGIC_defelem(y)
    MG_FLAGS = 0x02
      REFCOUNTED
    MG_OBJ = 0xcb2820
    SV = PV(0xcb2ed8) at 0xcb2820
      REFCNT = 1
      FLAGS = (POK,pPOK)
      PV = 0xce97f8 "akey"\0
      CUR = 4
      LEN = 10
  TYPE = y
  TARGOFF = 0
  TARGLEN = 1
  TARG = 0xce1a80
  FLAGS = 0
  SV = PVHV(0xcb8218) at 0xce1a80
    REFCNT = 2
    FLAGS = (SHAREKEYS)
    ARRAY = 0x0
    KEYS = 0
    FILL = 0
    MAX = 7

[download]
If this PVLV values is assigned to then it autovivifies the hash. If it is just read, it doesn't. It behaves kinda like a tied variable. Personally I don't like like this behaviour much, but it was added in 5.004, before my time as a p5 porter.

Dave.

In reply to Re^3: unexpected modify hash in a distance with grep { $_ } (inconsistent behaviour of aliasing) by dave_the_m
in thread unexpected modify hash in a distance with grep { $_ } by leszekdubiel

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.