Don't rely on "automatic quoting" too much... It's only garanteed to work for barewords. _$_ most definitely is not a bareword (= the kind of word one normally uses for variable and sub names). The same goes for the left hand side of =>, I'm sure _$_ will do the same magic trick there as it did in your code... 
my %hash = ( _$_ => 'foo' );

[download]
this shows the following error when ran like this: 
Can't call method "_" on an undefined value

The fact that it actually compiles, without any complaint, may be a bit unfortunate... But it's nothing out of the ordinary.

In reply to Re: Hashkey stringification ... sometimes. by bart
in thread Hashkey stringification ... sometimes. by dragonchild

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.