Since nobody else mentioned it, there's a key difference between your two code snippets. In your first, you appeared to be interested in making the caller's hash empty except for one key/value pair. Of course, you see now why that attempt failed in modifying the caller's hash at all. In your second, you simply added or replaced a single key/value pair, and left any other contents of the caller's hash alone. 
sub foo {
  my $ref = shift;

  %$ref = ( all => new );
}

[download]
This alternative will keep the caller's hash intact, but replace ALL of its contents with a completely new set of contents, consisting of one key/value pair. This will also work even if the caller's hash is actually a blessed reference; the existing blessing is not broken (desecrated?) by this method.

--
[ e d @ h a l l e y . c c ]

In reply to Re: Modifying values by reference: what works and what doesn't by halley
in thread Modifying values by reference: what works and what doesn't by markjugg

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.