Fair enough, but why does it work and print the value of $PI if I comment out the use of CONST in main entirely (i.e. main's last line)? Also adding a sub in XYZ and calling it from main works ok. Is it because it can live without unresolved subs until it actually has to call them? But it has to resolve each and every variable before execution (when using strict at least)?

The long story is that I ended using require because I have a module which overrides a core sub (sleep) via a BEGIN block. I need that to apply for ALL subsequent modules, mine or foreign, and I kind of reasoned that loading everything else at runtime (via require) will make sure that this is satisfied and sleep is overriden. Now I am not sure whether a module used via another module is using the overriden sleep version.

> Fair enough, but why does it work and print the value of $PI if I comment out the use of CONST in main entirely (i.e. main's last line)?

Strict has 3 effects, one (subs) is to restrict the use of barewords to only known subs. Constants are implemented as subs and need to be known at compile time.

See also "use subs "

This doesn't necessarily mean that the sub known at compile time is also the called sub, because resolution happens at run time. (Keep in mind that you can redefine subs dynamically, this will just update the reference in the symbol table)

Any sub called with & or trailing () is not a bareword anymore, and not effected by strict.

Another effect of strict (vars) is to enforce pre declaration of unqualified variables with my or our.

But your variables are fully qualified. And barewords are not variables (under strict).

Update

It all becomes much easier if you first understand how Perl works without strict, an why these restrictions need to be done.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

This subject is complicated. Please review the many answers to your previous post Copy a builtin sub to a different name and then override.
Bill

As Paladin pointed out, this is a compile-time-versus-runtime thing. Furthermore, since you want to fully qualify all the module symbol names anyway, I see no reason to bother with any Exporter stuff.

File XYZ.pm:

# XYZ.pm

package XYZ;

use strict;
use warnings;

# require Exporter;
#
# our @ISA=qw/Exporter/;
# our @EXPORT_OK = qw/$PI CONST/;

our $PI = 355/113;
use constant CONST => 42;

1;
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c:\@Work\Perl\monks\bliako>perl -le
"use strict;
 use warnings;
 ;;
 use lib '.';
 use XYZ;
 ;;
 print 'PI = ',   $XYZ::PI;
 print 'CONST = ', XYZ::CONST;
"
PI = 3.14159292035398
CONST = 42
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Update: BTW: Doing a require in a BEGIN block
    BEGIN { use lib '.';  require XYZ; }
would have had the same phasing effect as the use statement.

use const does it's trick by creating a sub that returns a then "constant" value. Use the function call parents to honour that fact in the context of the print statement at compile time.

print "CONST=" . XYZ::CONST() . "\n";
#                          ^^-here
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ok that's a neat trick

Others have hopefully helped you understand what the issue is. I'll add this:

use XYZ ();
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By specifying an explicit empty list at the end of the use statement, Perl will avoid importing anything from the module, but will still load it at compile time. So XYZ::CONST will work.

that's useful for me: I usually prefer using the full name of foreign subs anyway. thank you

>

print "PI=".$XYZ::PI."\n";
print "CONST=".XYZ::CONST."\n";
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Importing means aliasing symbols from an exporting namespace into an importing namespace.

Since you are fully referencing the exporting namespace XYZ, you won't need any importing.

Update

A simple do File would be enough.