I have an array reference with various undef values within it. I want to initialise all undef's so as a test I wrote 
#!/usr/local/bin/perl -w
use strict;

my $x = ['1','2',undef,undef,undef,'4'];
@$x = map { $_ ||= "" } @$x;

while (@$x) {
    print ">", shift @$x, "<\n";
}

### This gives
>1<
>2<
><
><
><
>4<

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That works as I wanted so I ported the code into my main program.... 
    $res = $dh->prepare($sql);
    $res->execute();

    my @dataset = ();
    # Iterate over result set building up the result set
    do {
        while (my $data = $res->fetch()) {
            if ($res->{syb_result_type} == CS_ROW_RESULT) {
                if (defined $data) {
                    @$data = map {$_ ||= ""} @$data;

[download]
This bombs instead with.. 
Modification of a read-only value attempted at TradeExtract.pl line 10
+3 (#1)

    (F) You tried, directly or indirectly, to change the value of a
    constant.  You didn't, of course, try "2 = 1", because the compile
+r
    catches that.  But an easy way to do the same thing is:

        sub mod { $_[0] = 1 }
        mod(2);

    Another way is to assign to a substr() that's off the end of the s
+tring.

Uncaught exception from user code:
    Modification of a read-only value attempted at TradeExtract.pl lin
+e 103.

[download]
I'm missing something simple here.. Help!

In reply to Modification of a read-only by Jonathan

Title:
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