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++;
}
[download]

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);
[download]

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
[download]

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);
[download]

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;
[download]

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
   }
}
[download]

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>) {
    }
}
[download]

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

---

Notes

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);
[download]

And the following are safe:

my @array = (1,2);
for (@array) {
  $_++;
}
[download]

my ($x,$y) = (1,2);
for ($x,$y) {
  $_++;
}
[download]

For an explanation of lists versus arrays I recommend the following:

• Re: Differ. array and List (there is no List)
• "List" Is a Four-Letter Word

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.