A little walkthrough

$dir="/home/yehuda/img/jpg/";
chop $dir;
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Here you hardcode a string and immedeatly chop off the last char. I cannot make any sense of that.

while (defined ($img=readdir DH)){
$img=~m/^(.*)\.(.*)\.(.*)/g;
$newname="$1.$3";
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Here you do not check if the regex matches. If there is file that does not meet your assumptions it will horribly fail.

push @names,$img;
push @newnames,$newname;
}
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I'm not sure why you don't rename the files immedeatly, but ok. i can live with that. Even if you really have to store the filenames somewhere i would use a hash for that. old name as key, new name as value.

close DH;
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close() closes a filehandle, not a dirhandle. use closedir();

foreach $name(@names){
$i=$i+1 ;
$newname_=$newnames[$i-1];
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Now that is the fun part. You add 1 to $i just to substract the 1 again when you use $i. Better to do the addition after the usage and leave out the substraction.

$path="$dir/$name";
$target="$dir/$newname_";
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Two unneccessary variables.

chomp($target,$path);
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Why chomp? there is no newline insight.

rename ($path,$target) or die "Could not rename:$!";
print "  $path   is now $target \n";
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I'd prefer a warning here instead of sudden death.

After all I doubt that you got this code from the cookbook. And if so, throw it into the trashcan. Slightly improved:

$dir="/home/yehuda/img/jpg";

opendir(DH,$dir)or die "FOO BAR! $!\n";

while ( readdir DH)
{
    if ( -f "$dir/$_" && $m/^(.*)\.(.*)\.(.*)/ )
    {
        warn "cannot rename $dir/$_\n"
            unless rename "$dir/$_", "$dir/$1.$3";
    }
}
closedir DH;
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