G'day nameofmyuser,

Welcome to the Monastery.

Perl provides a number of special blocks which are executed at various points during the lifetime of the program: "BEGIN, UNITCHECK, CHECK, INIT and END". You can use these to check your assumptions about what's happening at these various points.

[Warning: Do read the linked documentation, especially the begincheck example. Some of those blocks are executed in FIFO order; and some in LIFO order. It's not intuitively obvious which is which.]

Your assumptions appear to be correct; although, the phrase "@test array is allocated at compile time" could be misinterpreted. To be clear, in both of these:

my $var1;
my $var2 = 'val2';
[download]

The variables are declared at compile time; but no assignment occurs at this time. The "$var2 = 'val2'" part of that second statement occurs at runtime.

Here's an example using some of those special blocks. It's just a start: modify and extend as necessary.

#!/usr/bin/env perl -l

use strict;
use warnings;

my $hashref = { a => 1, b => 2 };
my @array = qw{c d e};

use constant {
    HR_ARRAY => [ sort values %$hashref ],
    AREF => \@array,
};

BEGIN { print join ' ', 'B1:', sort(keys %$hashref), @array }
BEGIN { print join ' ', 'B2:', @{+HR_ARRAY}, @{+AREF} }

INIT { @array = qw{f g h} }
INIT { print join ' ', 'I1:', sort(keys %$hashref), @array }
INIT { @array = qw{i j k} }
INIT { print join ' ', 'I2:', @{+HR_ARRAY}, @{+AREF} }

print join ' ', 'R1:', sort(keys %$hashref), @array;
print join ' ', 'R2:', @{+HR_ARRAY}, @{+AREF};
[download]

Output:

B1:
B2:
I1: f g h
I2: i j k
R1: a b c d e
R2: c d e
[download]

Using constant for references is generally a bad choice. It makes the token that points to the reference a constant; but, it does not make the values of the reference constants. Consider these:

$ perl -E 'use constant AREF => [1..4]; say "@{+AREF}";'
1 2 3 4

$ perl -E 'use constant AREF => [1..4]; say "@{+AREF}"; AREF = []'
Can't modify constant item in scalar assignment at -e line 1, at EOF
Execution of -e aborted due to compilation errors.

$ perl -E 'use constant AREF => [1..4]; say "@{+AREF}"; AREF->[2] = 99
+; say "@{+AREF}"'
1 2 3 4
1 2 99 4
[download]

In terms of workarounds: one of the Anonymous Monks showed a BEGIN block; you could potentionally use an INIT block with a variation of what my code showed for AREF; you could also just use two constants, something like this:

use constant { ONE => 1, ..., FOUR => 4 };
use constant NUM_AREF => [ ONE .. FOUR ];
[download]

[Note: I've used NUM_AREF which could probably be improved upon; however, HASHREFCONST is a poor, and highly misleading, choice for an arrayref constant: it will make subsequent code difficult to understand, and runs a high risk of being the source of coding errors.]

If you tell us how this code is intended to be used, we can probably offer more concrete suggestions.

See also: enum — a CPAN module which you might find useful.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.