The empty pattern //
If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead.

Yeah , and then what? What is your question?

The empty regex always matches

$ perl -Mre=debug -e " m//"
Compiling REx ""
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0
Matching REx "" against ""
   0 <> <>                   |  1:NOTHING(2)
   0 <> <>                   |  2:END(0)
Match successful!
Freeing REx: ""

$ perl -Mre=debug -e " $_ = q/fi/; m//"
Compiling REx ""
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0
Matching REx "" against "fi"
   0 <> <fi>                 |  1:NOTHING(2)
   0 <> <fi>                 |  2:END(0)
Match successful!
Freeing REx: ""
[download]

The empty regex always matches

Well, yes, and no. // is special, it serves as a shortcut, as if it's not the first regex you match, it signifies a repeat of the last regex that matched.

perlreref:

If 'pattern' is an empty string, the last successfully matched regex is used.

In your tests here, there was no previous match, so this rule doesn't apply.

I'm writing a scraper and sometimes I need to exclude some data. As I said, doing:

$css->{xlink} //= ' ';
[download]

works in this case but it just doesn't seem like the right way to do this.

@{$ret->{link}} = grep { ! /$css->{xlink}/ } map { $_->{href} } $dom->
+find($css->{link})->each if ($css->{link});
[download]

IMHO my post should have helped you as it showed that you can extend the expression in the grep codeblock.

But now we know a little more of your problem and I assume the following: If $css->{xlink} is defined and not empty you want to try a match. The otherwise clause is not obvious to me, but you can change it. E.g.

grep { $css->{xlink} && ! /$css->{xlink}/ }
[download]

I hope I understood it right.

McA

I'm writing a scraper and sometimes I need to exclude some data. ...

Well, that much (your desire) was clear from your original post, but its still not a question :)

If you want to filter based on a regex, and a space isn't adequate, what is adequate?

?? if( sometingTrue ){ grep filter } else { no grep filter }

Or were you after what McA wrote?

my @p = grep {
    defined $_ && $_ ne ''
} @arr;
[download]

McA