Part of your question, though you didn't realize is, is how use constant works.

use constant creates a subroutine of no arguments for the constant you want, which returns the desired value. You COULD do the same manually.

sub FOO(?) {10;}
sub BAR(?) {20;}
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While it does work in expressions, FOO is not recognized as being a subroutine, when you use it in a string. While there are ways to make it be interpolated within a real string, which may or may not work in the fat-arrow string, the more modern solution is to use Readonly instead of <c.constant</c>. Readonly gives you real scalars, arrays, hashes which can't be modified.

use Readonly
Readonly my $FOO => 10;
Readonly my @GAMES => ( 'tennis', 'crocket', 'crazy eights' );
Readonly my %NUMBERS 
    => ( consonents => [ qw( b c d f g h j k l m n p q r s t v w x z) 
+],
         vowels     => [ qw( a e i o u y ) ],
       );
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