Note that the module itself mentions the following:

For example, you can't say $hash{CONSTANT} because CONSTANT will be interpreted as a string. Use $hash{CONSTANT()} or $hash{+CONSTANT} to prevent the bareword quoting mechanism from kicking in. Similarly, since the => operator quotes a bareword immediately to its left, you have to say CONSTANT() => 'value' (or simply use a comma in place of the big arrow) instead of CONSTANT => 'value'.

The OP used &CONSTANT => 'value' which works but doesn't inline the constant (i.e. expand it during compile time).

ikegami pointed me to a different way which I found more pleasing than CONSTANT() which, as he rightly noted, leaks the internal implementation of constants.

use constant A => 12;
my %hash = ( (A) => 'twelve' ); # beautiful
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I wanted to verify it behaves exactly the same, so I tried running it through B::Deparse, B::Terse, and B::Concise.

m=Deparse
diff <(perl -MO=$m -e 'use constant A => 12; my %hash = ( A() => "twel
+ve")') \
     <(perl -MO=$m -e 'use constant A => 12; my %hash = ( (A) => "twel
+ve")')
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Terse needs some tweaking to skip the pointers:

m=Terse
diff <(perl -MO=$m -e 'use constant A => 12; my %hash = ( A() => "twel
+ve")' \
       | perl -pe 's/0x\w+/X/g') \
     <(perl -MO=$m -e 'use constant A => 12; my %hash = ( (A) => "twel
+ve")' \
       | perl -pe 's/0x\w+/X/g')
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But Concise shows a slight difference:

m=Concise
diff <(perl -MO=$m -e 'use constant A => 12; my %hash = ( A() => "twel
+ve")') \
     <(perl -MO=$m -e 'use constant A => 12; my %hash = ( (A) => "twel
+ve")')

7c7
< 4           <$> const[IV 12] s*/FOLD ->5
---
> 4           <$> const[IV 12] sP*/FOLD ->5
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From the documentation it seems the P just means that A was parenthesized. I can imagine this information could be valuable to Perl (e.g. in the LHS of an assignment, but the structures of scalar versus list assignments are much more different); but probably not in this case.

Update: Topics for meditation include other possible syntaxes (e.g. A ,=> 12), personal preferences, explanation of the Concise's output, etc.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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