The documentation explains that the + separates the print function from the parentheses ( and tells perl that these are not the parentheses wrapping the parameters of the print function.

That might satisfy you, but if you are further intersted you can use the B::Deparse module to ask perl how does it understand this code-snippet:

print +(stat $filename)[7];

We save that content in the plus.pl file and run perl -MO=Deparse plus.pl. The result is:

print((stat $filename)[7]);

and also

files/plus.pl syntax OK

As you can see the otherwise unnecessary + sign has disappeared but, instead of that perl added an extra pair of parentheses. These are the parens wrapping the parameters of the print function.

How does it work with say?

Now that we saw this with the print function, lets make the seemingly obvious change and replace print by say in the plus.pl file:

say +(stat $filename)[7];

run perl -MO=Deparse plus.pl, and the result is:

'say' + (stat $filename)[7];

What???

That's surprising, and for a few seconds you don't really know what does that mean, but then you remember that say is not part of perl by default. You need to tell somehow that you want the say function to be part of your language. For example by writing use 5.010;.

So we change the code to this:

use 5.010;

say +(stat $filename)[7];

run perl -MO=Deparse files/plus.pl and get the following result:

sub BEGIN {
    require 5.01;
}
no feature;
use feature ':5.10';
say((stat $filename)[7]);

That a lot of code, but at least we have our extra parens back wrapping the parameters of say.

Conclusion

B::Deparse can be useful when you need to know how perl understands a code snippet.

Check out the other articles about B::Deparse.