What does !! (double exclamation point) mean?

In the code below, from a blog post by Alias, I noticed the use of the double exclamation mark !!. I was wondering what it meant and where I could go in the future to find explanations for Perl syntax like this. (Yes, I already searched for !! at pe开发者_StackOverflow中文版rlsyn).

package Foo;
 
use vars qw{$DEBUG};
BEGIN {
    $DEBUG = 0 unless defined $DEBUG;
}
use constant DEBUG => !! $DEBUG;
 
sub foo {
    debug('In sub foo') if DEBUG;
 
    ...
}

UPDATE

Thanks for all of your answers.

Here is something else I just found that is related The List Squash Operator x!!

It is just two ! boolean not operators sitting next to each other.

The reason to use this idiom is to make sure that you receive a 1 or a 0. Actually it returns an empty string which numifys to 0. It's usually only used in numeric, or boolean context though.

You will often see this in Code Golf competitions, because it is shorter than using the ternary ? : operator with 1 and 0 _{($test ? 1 : 0)}.

!! undef  == 0
!! 0      == 0
!! 1      == 1
!! $obj   == 1
!! 100    == 1

undef ? 1 : 0  == 0
0     ? 1 : 0  == 0
1     ? 1 : 0  == 1
$obj  ? 1 : 0  == 1
100   ? 1 : 0  == 1

not-not.

It converts the value to a boolean (or as close as Perl gets to such a thing).

Because three other answers claim that the range is "0" or "1", I just thought I'd mention that booleans in Perl (as returned by operators like ==, not, and so on) are undef and 1, not 0 and 1.