Question

I’m writing an app in Perl with several modules. I want to write some global constants that will be visible from everywhere, like this:

#Constants.pm $h0 = 0; $scale = 20; 

And then use them without qualifying with main:: or Constants:: in several modules. However, if I write use Constants; in more than one module, they only get imported into one namespace. Is there any way around this?

I’m using the latest ActivePerl.

Answer 1

You can put this at the top of Constants.pm:

package main; 

In this case all the variables you define will be in the main namespace:

$main::x 

or if you’re feeling brave:

package; 

In this case all the variables you define will be in an empty namespace:

$::x 

Note that using package with no namespace is discouraged, and will apparently be deprecated in some versions of Perl. See the quote below.

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Quoting from man perlfunc:

         package NAMESPACE        package Declares the compilation unit as being in the given                namespace.  The scope of the package declaration is                from the declaration itself through the end of the                enclosing block, file, or eval (the same as the 'my'                operator).  All further unqualified dynamic identifiers                will be in this namespace.  A package statement affects                only dynamic variables--including those you've used                'local' on--but not lexical variables, which are cre?                ated with 'my'.  Typically it would be the first decla?                ration in a file to be included by the 'require' or                'use' operator.  You can switch into a package in more                than one place; it merely influences which symbol table                is used by the compiler for the rest of that block.                You can refer to variables and filehandles in other                packages by prefixing the identifier with the package                name and a double colon:  $Package::Variable.  If the                package name is null, the 'main' package as assumed.                That is, $::sail is equivalent to $main::sail (as well                as to $main'sail, still seen in older code).                 If NAMESPACE is omitted, then there is no current pack?                age, and all identifiers must be fully qualified or                lexicals.  However, you are strongly advised not to                make use of this feature. Its use can cause unexpected                behaviour, even crashing some versions of Perl. It is                deprecated, and will be removed from a future release.  

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Edit: This question might be helpful as well: How do I use constants from a Perl module?