You tried, directly or indirectly, to change the value of a constant. This type of error frequently occurs at a distance and is difficult to trace. The magic of $_ is often involved, but is not responsible.

Common causes:

  1. Treating a loop variable as an lvalue
  2. Modifying $_ inside foreach, map or grep
  3. Modifying elements of @_ directly
  4. Modifying $a or $b inside sort
  5. Autovivifying $a or $b inside sort
  6. Modifying an unlocalized $_

Treating a loop variable as an lvalue

for my $x (1,2) { $x++; }
In this example $x is aliased to the constant '1', so when the loop body attempts to increment $x an error is triggered. See Lists With Constant Values for more details.

Modifying $_ inside foreach, map or grep

for (1,2) { chomp; } for ("foo", @list) { s/foo/bar/; } @array = map { $_++ } (1,2); @array = grep { $_++ } (1,2);
In all of these examples $_ is aliased to a constant, and when the loop body attempts to modify $_ an error is triggered. See Lists With Constant Values for more details.

Modifying elements of @_ directly

sub incr { $_[0]++; } my $n = 1; incr($n); # good incr(1); # bad
Modifying elements of @_ directly allows you to modify the variable that was passed to the function. For example, in the above example $n is now 2. But an error will occur when a constant is passed, as in the second call.

Modifying $a or $b inside sort

@array = sort { $a++ } (1,2);
It is permissible (but ill-advised) to modify $a and $b within sort. However, modifying a constant that is aliased to $a or $b is still an error.

Autovivifying $a or $b inside sort

my @bad; $bad[0] = [1]; $bad[2] = [2]; @bad = sort {$a->[0] <=> $b->[0]} @bad;
The variables $a and $b are aliased to each item in the list being sorted, and as such modifying them is possible - but will cause an error if the current element is unmodifiable. A common cause of this is sorting an array of references when where the list has a gap. In this situation $a will be undef, and autovivification by dereferencing will trigger an error.

Modifying an unlocalized $_

for (1,2) { my $data = prompt_user(); } sub prompt_user { print "Enter a number\n"; while (<STDIN>) { # Do stuff } }
This example will cause an error because the for loop aliases $_ to the literal '1', and then calls prompt_user which attempts to read a line from STDIN and store it in $_ - which is still aliased to '1'.

The error will also occur in this simplified scenario:

for (1,2) { while (<STDIN>) { } }

Guidelines to avoid read-only errors:

  1. Don't treat loop variables as a lvalue if there is any chance a constant value will be included
  2. Don't modify an unlocalized $_
  3. Don't modify $_ inside map or grep
  4. Don't modify $a or $b inside sort
  5. Don't dereference $a or $b inside sort without checking that they exist and are references
  6. Don't use $_ for the loop variable unless it is a very trivial loop
  7. Don't modify elements of @_ directly


Lists With Constant Values

Within the context of this document the important thing to understand is what expressions result in modifiable objects. The following expressions have constants in them:
$_++ for (1,2); $_++ for (1,@array); @array = map {$_++} (1,@array);
And the following are safe:
my @array = (1,2); for (@array) { $_++; }
my ($x,$y) = (1,2); for ($x,$y) { $_++; }
For an explanation of lists versus arrays I recommend the following: