Using $h{x} is sufficient to autovifiy a key/value pair if there is no key/value pair for the key 'x' in the hash %h.

In the first case you give, the key 'x' is added to the hash (the $h{x} bit does that) with the value undef. The %{...} then tries to dereference the undef and generates the error that you see.

The second case is equivalent to:

my $ref = undef;
keys %{$ref}; # or: keys %$ref;

[download]

which, somewhat surprisingly, is fine and is about the same as asking for the keys of an empty hash. I guess this is an element of DWIM coming into play - if you ask for the keys of a hash reference that you haven't gotten around to assigning any key/value pairs to yet, then a list of not keys (and no errors generated) is a pretty reasonable thing to do.

Perl is environmentally friendly - it saves trees

In reply to Re^3: Keys() required to autovivify? by GrandFather
in thread Keys() required to autovivify? by jrw

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.