How can I qualify a variable as const/final in Perl?

Question

For example, in situations like below, I do not want to change the value of $infilename anywhere in the program after initialization.

my $infilename = "input_56_12.txt";
open my $fpin, '<', $infilename
    or die $!;

...
print "$infilename has $result matches\n";

close $fpin;

What is the correct way to make sure that any change in $infilename results in not just warnings, but errors?

Answer 1

use Readonly;
Readonly my $infilename => "input_56_12.txt";

Or using the newer Const::Fast module:

use Const::Fast;
const my $infilename => "input_56_12.txt";

Answer 2

use constant INPUT_FILE => "input_56_12.txt";

Might be what you want. If you need to initialize it to something that may change at run time then you might be out of luck, I don't know if Perl supports that.

EDIT: Oh, look at eugene y's answer, Perl does support that.