Just to clarify (for all respondants) I'm investigating a bug in Data::Rmap where the following loops off forever: 
perl -MData::Rmap=rmap_hash -le 'rmap_hash {print keys %{$_->{a}} } ({
+a=>{1,2}})'

[download]
I'm not certain, but I suspect that the keys call interacts with this line, from Data::Rmap: 
push @return, $self->_rmap($_) for values %$_;

[download]
Since the module is for general use, XS or tie-ing aren't really options. Also, $_ is aliased during the callback so that things, including hashes can be changed (sometimes leading to different disasters). This further constrains the solution space.

Thanks for your tips,

Brad

Update: I think autovivification is to blame 

# added exists check
perl -MData::Rmap=rmap_hash -le 'rmap_hash {print keys %{$_->{a}} if e
+xists $_->{a} } ({a=>{1,2}})'
1

[download]

In reply to Re^2: Perl leve control of hash iterators by bsb
in thread Perl level control of hash iterators by bsb

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.