Why does Perl complain when I use a hash reference with constant.pm?

Question

I have perl, v5.6.1 built for MSWin32-x86-multi-thread Binary build 638 provided by ActiveState.

I am working on a Perl script where I have declared constants that are used later for comparison purposes. For some reason, I am getting an error that states something along the line of Constant name has invalid characters at script's line 31 (The line right after the use constant clause in the code below). I checked and found out that '_' (underscores) in Constant name is a legit character. I also tried to change '0.00' to just '0' to see if that was the cause but I got the same error. I am not sure what I am doing wrong. Anyone know why the compiler does not like this?

Thanks!

Here is the Code:

use constant {
MIN_NET_DLR => 0.00,
MAX_NET_DLR => 99.99,
MIN_SUM_DLR => 0.00,
MAX_SUM_DLR => 999.99,
MIN_UNITS => 0,
MAX_UNITS => 99,
MIN_SUM_UNITS => 0,
MAX_SUM_UNITS => 999,
PCT_THRES_AO => 1,
PCT_THRES_TRANS_CUST_BI => 20,
PCT_THRES_CUST => 3,
};

PROBLEM:

The problem is that the version of constant provided by perl 5.6.1 does not support hash reference.

SOLUTION:

Use the regular declaration for constants. Hence, the declaration will look as follows:

use constant MIN_NET_DLR => 0.00;
use constant MAX_NET_DLR => 99.99;
use constant MIN_SUM_DLR => 0.00;
use constant MAX_SUM_DLR => 999.99;
use constant MIN_UNITS => 0;
use constant MAX_UNITS => 99;
use constant MIN_SUM_UNITS => 0;
use constant MAX_SUM_UNITS => 999;
use constant PCT_THRES_AO =>    1;
use constant PCT_THRES_TRANS_CUST_BI => 20;
use constant PCT_THRES_CUST => 3;

Thanks @leon for the solution as well as others who chimed in with their inputs.

UPDATE: Another (more elegant) solution is to update your Perl version to the one that supports hash reference in declaring constants.

Answer 1

Works for me. Are you sure you don't have any control characters in that code somewhere?

Answer 2

Remove the last comma at the end of this line:

PCT_THRES_CUST => 3,

That could be the issue.

Answer 3

The problem is that you are using a hash reference. The version of constant provided by perl 5.6.1 does not support that. You'll either have to remove the braces or upgrade your constant.

Answer 4

It works fine for me on perl5.8 but doesn't work on perl5.6 . The error message is:

Can't define "HASH(0x80c05d8)" as constant (name contains invalid characters or is empty)

Seems like an old-version issue, probably the issue that Leon described.

Answer 5

Your old perl probably does not support the syntax where you declare multiple constants in one statement. Try defining each constant separately like this...

use constant MAX_NET_DLR => 99.99;

Answer 6

I would use the Readonly module.

The disadvantage of constant is, that it creates a function within the callers namespace which returns a constant value.

Readonly can be used to declare scalars, arrays and hashes readonly (real constants).