% perl -MData::Dumper
$c->[3] = 3;
print Dumper( $c );
for ( @$c ) { $_++ }
print Dumper( $c );
__END__
$VAR1 = [
          undef,
          undef,
          undef,
          3
        ];
$VAR1 = [
          1,
          1,
          1,
          4
        ];

##</code><code>##

% perl -MData::Dumper
$c->[3] = 3;
print Dumper( $c );
map { $_++ } @$c;
print Dumper( $c );
__END__
$VAR1 = [
          undef,
          undef,
          undef,
          3
        ];
Modification of a read-only value attempted at - line 3.