Why is Perl foreach variable assignment modifying the values in the array?

Question

OK, I have the following code:

use strict;
my @ar = (1, 2, 3);
foreach my $a (@ar)
{
  $a = $a + 1;
}

print join ", ", @ar;

and the output?

2, 3, 4

What the heck? Why does it do that? Will this always happen? is $a not really a local variable? What where they thinking?

Answer 1

Perl has lots of these almost-odd syntax things which greatly simplify common tasks (like iterating over a list and changing the contents in some way), but can trip you up if you're not aware of them.

$a is aliased to the value in the array - this allows you to modify the array inside the loop. If you don't want to do that, don't modify $a.

Answer 2

See perldoc perlsyn:

If any element of LIST is an lvalue, you can modify it by modifying VAR inside the loop. Conversely, if any element of LIST is NOT an lvalue, any attempt to modify that element will fail. In other words, the foreach loop index variable is an implicit alias for each item in the list that you're looping over.

There is nothing weird or odd about a documented language feature although I do find it odd how many people refuse check the docs upon encountering behavior they do not understand.