++ for restricted hashes. But as pointed by ikegami, they do not catch the exception at compile time.

There might be a source filter somewhere on CPAN that does what you want, if you're ok with using a source filter. Parsing the code for /[$](\w+)[{](\w+)[}]/ and making sure that $2 is a valid key for hash $1 would probably be easy and safe enough.

Otherwise, there's the solution of using constants. Either by using an array instead of the hash: 

use strict;
use warnings;
use constant {
  FOO => 0,
  BAR => 1,
  BAZ => 2
};

$var = BAZ;

my @array;
say $array[FOO];  # OK
say $array[BAH];  # Bareword "BAH" not allowed while "strict subs" in 
+use
say $array[$var]; # OK

[download]
Or never allowing the bareword interpretation for a hash key by adding a + (there's no way to disable ALL barewords is there?) 
use strict;
use warnings;
use constant {
  FOO => "foo",
  BAR => "bar",
  BAZ => "baz"
};
my %hash;
say $hash{+FOO};  # OK
say $hash{+BAH};  # Bareword "BAH" not allowed while "strict subs" in 
+use
say $hash{+shift} # Bonus

[download]

In reply to Re: strict "vars" mode for hash key literals? by Eily
in thread strict "vars" mode for hash key literals? by perlancar

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