I'm wondering if you are mis-understanding =>. It is not an assignment, but a "fat comma". It allows a bareword on its left-hand side, but other than that is just a comma. This means you are invoking the comma operator in list context, which results in the rightmost value. So:
my %a = map { $a{$_} => 1 unless undef, 0..5 } 0..11;

[download]
Is the same as
my %a = map { 1 unless undef, 0..5 } 0..11;

[download]
In fact, if you had run with use strict; and use warnings; things may have been clearer because that code does not actually compile. It requires the %a to be declared first (since it is used in the map on the rhs). When I run this:
use strict;
use warnings; 

use Data::Dumper qw/Dumper/;
my %a;
#%a = map { $a{$_} => 1 unless undef, 0..5 } 0..11;
#%a = map { $a{$_} , 1 unless undef, 0..5 } 0..11;
%a = map { 1 unless undef, 0..5 } 0..11;
print Dumper \%a;

[download]
I get (5.10):
 
Use of uninitialized value in range (or flip) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
Use of uninitialized value in range (or flop) at C:\gash.pl line 10.
$VAR1 = {
          '11' => 12,
          '1' => 2,
          '3' => 4,
          '7' => 8,
          '9' => 10,
          '5' => 6
        };

[download]

In reply to Re: something funny with hashes? by cdarke
in thread something funny with hashes? by Pete_I

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.