what does perl operator “||=” do?

Question

What does the operator ||= do in perl?

to be more specific if you have a code like:

my ($my_link);
$my_link  ||= DownloadF($file,\'l\') if          


        

Answer 1

It assigns only if variable evaluates to false value. In each of your example lines, $my_link will only be assigned if the condition $s->{..} is true.

Answer 2

it means if $my_link is nil/has no value, then assign it this value with = (value)

if $my_link already has a value, then it don't do anything

Answer 3

Perl supports lots of assignment operators. ||= is just a logical or (complete with shortcircuit,) assignment.

So essentially what you're looking at is:

if ($s->{_l}) {
  $my_link = $my_link || DownloadF($file,'l');
}

So if $my_link evaluates to some true value then $my_link will be assigned to itself (a no-op essentially), otherwise the result of DownloadF is assigned.

Other assignment operators supported by perl:

 **= += *= &= <<= &&=
-= /= |= >>= ||=
.= %= ^= //=
x=

Answer 4

If $my_link is false (empty string, 0 or undef) store DownloadF($file,'l') into $my_link

This construct has always had problems when used to assign a default value (what if you want $my_link to be zero)