2. like this return 1 if $_ !~ /\s+/; ?

No, because then you would have to perform a second match, which is unneeded. The s// operator returns the number of substitutions made, and 0 is perl's value for false.

    if ($_ =~ s/ /_/g) {
        # rename
    } else {
        return 1
    }
[download]

However, according to the docs for File::Find the return value of the wanted() function is ignored, so just write:

    if ($_ =~ s/ /_/g) {
        # rename
    }
[download]

Thank you, now I get!

Could you please tell why performing second match is bad? It can cause some errors? Or it spends resources of the machine? Or is it just a bad style of programming?

Because it's unnecessary. Why would you want to perform operations in your code that weren't necessary? Would you write a hello world program like this:

use strict;
use warnings;
use 5.012;


if ("hello world" =~ /hello/) {
    print 'hello';
}

if ("hello world" =~ /world/) {
    print " world\n";
}

--output:--
hello world
[download]

That program unnecessarily uses program resources, and therefore it is bad programming style.