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Perl: constants and compile-time vs runtime hash lookups?

In Perl 5.26, constant-based hash lookups appear to be resolved at compile-time, not runtime. How can I enforce it to be resolved at runtime?

Consider the following reduced testcase, boiled down from a hash-based state-machine I was trying to write, where the key is the state identifier and the value is the state function.

use constant {
STATE_1 => 1,
STATE_2 => 2,
};

my %fsm;

%fsm = (
STATE_1, sub {
$fsm{STATE_2}->(@_);
return STATE_2;
},
STATE_2, sub {
return STATE_1;
}
);

my $state = STATE_1;

$state = $fsm{$state}->();


Note that in STATE_1, I'm trying to call the STATE_2 function.

STATE_1

STATE_2

However, at runtime I get this:

Can't use an undefined value as a subroutine reference at ./self-reference-hash.pl line 15.


Which indicates that the $fsm{STATE_2}->(@_); line in STATE_1 is undefined. And indeed, at the time where this line first appears, the STATE_2 function isn't defined yet, but I was counting on hash lookups being resolved at runtime.

$fsm{STATE_2}->(@_);

STATE_1

STATE_2

If I instead replace $fsm{STATE_2}->(@_); with my $tmp = STATE_2; $fsm{$tmp}->(@_); then it works as expected, which seems hacky.

$fsm{STATE_2}->(@_);

my $tmp = STATE_2; $fsm{$tmp}->(@_);

Is there a cleaner way to do this?

use Const::Fast; const my $STATE_1 => 1;

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