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

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 $!;

...
prin开发者_StackOverflow中文版t "$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?

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";

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.

Another popular way to create read-only scalars is to modify the symbol table entry for the variable by using a typeglob:

*infilename = \"input_56_12.txt";

This only works for global variables ("my" variables have no symbol table entry).