Alternatively, set up a hash with your error messages:

use strict;

use constant ERR => 'Some error happened.';
use constant OK  => 'All is fine.';

my %messages = (
    'ERR' => ERR(),
    'OK'  => OK(),
);

my $res = some_func();
print $messages{ $res }, "\n";
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Alternatively, you can invite the demons of symbolic references into your house, but they will eat your children:

use strict;

use constant ERR => 'Some error happened.';
use constant OK  => 'All is fine.';

my $res = some_func();
{
    no strict 'refs';
    print &$res(), "\n";
};
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