I'm really having a hard time putting responses where they need to be. Since my re-working of what aaron posted and what AM posted are catching in the same place, I hope that putting the responses here doesn't offend.

I had two big problems in the original post that have now been corrected: single quotes around the zero for false and calling for a new DirPP while in the loop that downloads the images.

Now I'm trying to integrate the slicker versions of DirPP and stumble on the same error despite which version I try:

$ perl tg2.pl
Unrecognized LWP::UserAgent options: autodie at tg2.pl line 7
Use of uninitialized value $dir in string at tg2.pl line 16.
Use of uninitialized value $dir in string at tg2.pl line 16.
...
Use of uninitialized value $dir in string at tg2.pl line 16.
^C
$ cat tg3.pl
#!/usr/bin/perl -w
use strict;
use WWW::Mechanize;
use LWP::Simple;
use feature qw/ state /;
my $domain = 'http://www.yahoo.com';
my $m = WWW::Mechanize->new( qw/ autodie 1 /); 
$m->get( $domain);
my $counter = 0;
my @list = $m->images();
my $dir = &DirPP;
for my $img (@list) {
    my $url = $img->url_abs();
    $counter++;
  
    my $filename = "$dir". "/image_". "$counter";  #line 16
    getstore($url,$filename) or die "Can't download '$url': $@\n";
}
sub DirPP {
    use Errno qw/ EACCES /; # permission denied
    my $word    = "site";
    my $counter = 1;
    my $name ;
    my $made = 0;
    while(1){
        $name = sprintf '%s%3d', $word, $counter;
        last if not $made = mkdir $name, 0755 ;
        die $! if $! == EACCES;
    }
    return $name if $made;
    return;
}
$
[download]

From the look of the output, the subroutine is being called many times, but to my eye, I call it only once, and that is before I enter the loop that assigns a value to $dir . (scratches head) That was aaron's version, but I induce the same behavior with AM's version:

 $ perl tg2.pl
Unrecognized LWP::UserAgent options: autodie at tg2.pl line 7
Use of uninitialized value $dir in string at tg2.pl line 15.
Use of uninitialized value $dir in string at tg2.pl line 15.
Use of uninitialized value $dir in string at tg2.pl line 15.
Use of uninitialized value $dir in string at tg2.pl line 15.
Use of uninitialized value $dir in string at tg2.pl line 15.
Use of uninitialized value $dir in string at tg2.pl line 15.
Use of uninitialized value $dir in string at tg2.pl line 15.
Use of uninitialized value $dir in string at tg2.pl line 15.
^C
$ cat tg2.pl
#!/usr/bin/perl -w
use strict;
use WWW::Mechanize;
use LWP::Simple;
use feature ':5.10';
my $domain = 'http://www.yahoo.com';
my $m = WWW::Mechanize->new( qw/ autodie 1 /); 
$m->get( $domain);
my @list = $m->images();
my $counter = 0;
my $dir = &DirPP;
for my $img (@list) {
    my $url = $img->url_abs();
    $counter++;
    my $filename = "$dir". "/image_". "$counter";
    getstore($url,$filename) or die "Can't download '$url': $@\n";
}

    sub DirPP {

        use Errno qw/ EACCES /; # permission denied
        state $counter2 = 1;
        my $word    = "site";
        my $name ;
        my $made = 0;
        while(1){
            $name = sprintf '%s%3d', $word, $counter2;
            last if not $made = mkdir $name, 0755 ;
            die $! if $! == EACCES;
        }
        return $name if $made;
        return;
    }



$
[download]

So, I'm stumped and out of guesses. That means I have no excuse to stay on the computer...off to the world of work..., happy tuesday,

The error message isn't coming from within your subroutine; it's coming from within your loop through the images, at line 15. It's telling you that the $dir variable is undefined on each trip through the loop, although you're correct that you only assigned to it once, before the loop.

I only have time for a quick glance right now, but I suspect that your "last if not $made" logic has gotten things a little confused. To me, that says to break out of the loop if the directory was NOT made. If it was NOT made (which would be the case if your first attempt at picking a name picked one that already existed), then $made will be false, so your first return will not trigger, and your second return will return undef, which is assigned to $die. You probably want "last if $made", because you want to break out of the loop once the directory is created, right?

Aaron B.
My Woefully Neglected Blog, where I occasionally mention Perl.

Thx aaron and AM, I've got versions working now that are pretty close to what you prescribed:

$ perl tg4.pl
site_1 
site_2 
site_3 
site_4 
site_5 
site_6 
site_7 
site_8 
site_9 
site_10 
site_11 
site_12 
site_13 
site_14 
site_14 
$ cd site_14
$ ls
image_1   image_2   image_3   image_4   image_5   image_6   image_7   
+image_8
...
$ cd ..
$ cat tg4.pl
#!/usr/bin/perl -w
use strict;
use WWW::Mechanize;
use LWP::Simple;
use feature ':5.10';

# get information about images
my $domain = 'http://www.yahoo.com';
my $m = WWW::Mechanize->new(); 
$m->get( $domain);
my @list = $m->images();

# create new folder and download images to it.
my $counter = 0;
my $dir = &DirPP;
for my $img (@list) {
    my $url = $img->url_abs();
    $counter++;
  
    my $filename = "$dir". "/image_". "$counter";  #line 15
    getstore($url,$filename) or die "Can't download '$url': $@\n";
}
sub DirPP {
    use Errno qw/ EACCES /; # permission denied
    my $word    = "site";
    my $counter2 = 1;
    my $name;
    my $made = 0;
    while(1){
        $name = "$word" . "_" . "$counter2";
        $counter2++;
        print "$name \n";
        last if $made = mkdir $name, 0755 ;
        die $! if $! == EACCES;
    }
    print "$name \n";
    return $name if $made;
    return;
}
[download]

Again, I'm looking for tips on style or cautions for robustness. Since one can now demonstrate two ways to do this task, reasonable people could disagree on the final touches, and the discussion might help me to understand these routines better. Here's the one that uses a state variable. Do I use it correctly?

 $ perl tg5.pl
site_1 
site_2 
site_3 
site_4 
site_5 
site_6 
site_7 
site_8 
site_9 
site_10 
site_11 
site_12 
site_13 
site_14 
site_15 
site_16 
site_16 
$ cat tg5.pl
#!/usr/bin/perl -w
use strict;
use WWW::Mechanize;
use LWP::Simple;
use feature ':5.10';
use feature qw/ state /;
state $counter3 = 1;

# get information about images
my $domain = 'http://www.yahoo.com';
my $m = WWW::Mechanize->new(); 
$m->get( $domain);
my @list = $m->images();

# create new folder and download images to it.
my $counter = 0;
my $dir = &DirPP;
for my $img (@list) {
    my $url = $img->url_abs();
    $counter++;
  
    my $filename = "$dir". "/image_". "$counter";  #line 15
    getstore($url,$filename) or die "Can't download '$url': $@\n";
}
sub DirPP {
    use Errno qw/ EACCES /; # permission denied
    my $word    = "site";
    my $name;
    my $made = 0;
    while(1){
        $name = "$word" . "_" . "$counter3";
        $counter3++;
        print "$name \n";
        last if $made = mkdir $name, 0755 ;
        die $! if $! == EACCES;
    }
    print "$name \n";
    return $name if $made;
    return;
}
[download]

Yeah, autocheck should have been really called autodie instead, my bad