Question

While trying to understand how refp’s solution for random hash value selection works, I noticed something strange.

With repeated calls to the following Perl script, I consistently found that the first result returned was the same. The subsequent values returned were random:

use strict;
use warnings;
use 5.010;

my %hash = map { $_ => ord $_ } 'a' .. 'z';

say( (@_=%hash)[1|rand@_] ) for 1 .. 10;       # First value always 119

Interestingly, the following does not suffer from this issue:

sub random_value { ( @_ )[ 1 | rand @_ ] }

say random_value %hash for 1 .. 10;            # First value is random

Removing the references to @_ also remedies the problem:

say( (%hash)[1|rand keys %hash] ) for 1 .. 10; # First value is random

This has been tested on Windows (ActivePerl 5.14.2).

On the surface, it looks like setting @_ has something to do with it, but I’m not sure. Can anyone shed some light on what’s happening here?

---

EDIT

I thought this question was answered until refp provided an update. Why does the arrayref form not suffer from the same issue discussed above? :

[@_=%hash]->[1|rand@_] for 1 .. 10;            # First value is random

Answer 1

I suspect there is a race condition where @_ is not defined in the first loop iteration.

say( (@_=%hash)[1|rand@_] ) for 1 .. 10; 

Will become

say( (@_=%hash)[1|rand ()] ) for 1 .. 10; 

It escapes warnings because @_ is a predeclared variable. As you will notice:

say( (my @a=%hash)[1|rand@a] ) for 1 .. 10;   

Will crash and burn because @a is not defined in the postscript.

Update:

[@_=%hash]->[1|rand@_] for 1 .. 10;    

Is not any different. It is still bad practice to use a variable in the same statement that you assign it. The difference, I am guessing, is that the precedence is somewhat altered, so that the assignment is evaluated first.