If you have a bareword between ${}, or a literal string, that's taken as the name of a variable ("${STRING}" is the same as "$STRING").

It appears to be more complicated than that:

  use strict ; use warnings ;

  my $x = 57 ;
  print "\${x}   = '", ${x}, "'\n" ;
  print "\${'x'} = '", ${'x'}, "'\n" ;
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gives:

  ${x}   = '57'
  Can't use string ("x") as a SCALAR ref while "strict refs" in use ..
+.
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Turning off "strict refs" gives:

  ${x}   = '57'
  Use of uninitialized value in print ...
  ${'x'} = ''
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Replacing my $x by our $x, gives:

  ${x}   = '57'
  ${'x'} = '57'
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The interpretation of the bareword case doesn't look desperately useful to me... Anybody know why Perl doesn't treat it as a subroutine/constant ?

Sadly, this wouldn't help "${CONSTANT}" but it would mean that @{REF_ARRAY_CONSTANT} would do what might be expected.