In the following code, the second call to use_hash fails:

use strict;
use warnings;

$|=1;


sub empty_list {
  @{ [] }
}

sub use_hash {
  my (%h) = @_;
  print(join(',',keys %h),"\n");
}

use_hash(foo1 => undef, bar1 => 5);

print "not defined\n" unless defined ((empty_list)[0]);

use_hash(foo2 => (empty_list)[0], bar2 => 5);
[download]

The ouput of the program is as follows:

bar1,foo1
not defined
Odd number of elements in hash assignment at C:\tmp\emptylist_a.pl lin
+e 12.
foo2,5
[download]

We see that the first use_hash works. We also see that (not surprising) an empty list indexed by [0] yields undef. I would have expected the 2nd call to use_hash being the same as the first: Instead of passing undef<c> explicitly to key <c>foo2, I "generate" it by taking the first element of an empty list. However, the latter does not work.

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