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;
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Is the same as

my %a = map { 1 unless undef, 0..5 } 0..11;
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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;
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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
        };
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