Try Scalar::Readonly.  It's a tiny module, and variables made readonly with it are significantly faster than those of Readonly (because the latter uses the relatively slow tie mechanism).

But note that readonlyness can just as easily be turned off again, because the module just toggles Perl's internal readonly flag.  In other words, it's not the right tool to keep intentional evildoers from messing with your variables...

my $someVar = "someValue";
sub get_cnst {
    return $someVar;
}
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In this way, the variable is readable but not writable from outside the module. Is there a particular reason it needs to be a simple scalar?

Tried so many times so I forgot to mention additional requirements, sorry.

I don't like using getter because there are several scripts which uses module and several modules also, so number of getters and calls to them becomes big. Calls costs time and my task is optimisation so I would like to avoid getters.

By the way, I found that constants in Perl actually implemented by subs, so each mention of constant is actually call to sub which returns value. As for me this looks like having getters. So, looks like I don't need constants really. I need some way to define readonly object without use of additional modules and other time-spending things.

If you want it to be initialized once at run time and universally accessible for the remainder of the script's life, you could also accomplish this using a closure:

#!/usr/bin/perl
use strict;
use warnings;

sub definer {
    my $value = shift;
    sub outputter {
        no warnings "closure";
        print "$value\n";
    }
}

definer(5);
outputter();
definer(6);
outputter();
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If you are optimizing, do some profiling first to make sure that you are actually optimizing where it is needed - check out Devel::NYTProf. If you are worried about overhead introduced by function calls, it's possible Perl is not the best tool for this job or that you should delve into perlxs. But profile first and often.

How does a module cost you time? I really doubt that you have profiled your script to determine that using modules is what costs you time.

Then you want Readonly, not a constant.

My task is optimisation so I would like to make code as simple as possible, without use of additional modules and other time-spending things.

Thank you for info, I'll check if it fits my case.

my $CNST = $someVar;
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What I'm trying to do is having some object which is defined in the module and which can be used from script which uses this module but in readonly mode.

The $someVar = "someValue" assignment runs too late.

Anyway there's nothing preventing you from using an expression in the "use constant" statement:

use constant CNST => ($ENV{DEBUG} ? 1 : 0);
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You just have to keep in mind that the use statement is executed in "compile time" and therefore you can't use values computed at "run time". On the other time the "compile time" is just before the "run time", not days or months before as with C or something else that lets you compile your program and store the compiled version. Even in case you use PAR or PerlApp or Perl2Exe!