What you're seeing is the effect of "autovivification". It's not the print that's doing this, it's the hash access. $hash{'key0'}{'key1'} means you're asking Perl what's stored in the hashref at the key key0 in %hash, but since that doesn't exist, Perl infers from $hash{key0}{...} that you want $hash{key0} to be a hashref, so it creates it for you. The same thing happens a level deeper at $hash{'key0'}{'key1'}{'key2'} - note how it doesn't create key2 for you.

If you want to avoid this in core Perl, then first note that autovivification, as a rule of thumb, happens in "lvalue" context, that is, in places where a value could be assigned to the hash. This is true in sometimes surprising contexts, such as for loops, because the loop variable is an alias to the values being looped over. If you want to avoid autovivification in such places, you need to use exists to check for the existence of hash keys before accessing them. An alternative is to use no autovivification from CPAN.

In reply to Re: "print" of nonexistent element is actually altering a hash by haukex
in thread "print" of nonexistent element is actually altering a hash by larrymenard

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.