Time for code review ;)

###########################################################
### not gonna complain about strict, for whatever reason
use MP3::Info;

die "Must use parameters and seperator\n (SEPERATOR ARTIST TITLE YEAR 
+ALBUM GENRE COMMENT)" unless @ARGV;
$sep=shift;
@tokens=@ARGV;
while(<*.mp3>) # while is wrong as you move files around, 
               # should be for (grep /\.mp3$/i, <*>)
               # to insure getting files like "bla.MP3" 
### indent your code!
{
  my $tag=get_mp3tag($_) or (push(@errors,"No tag for $_\n")   and nex
+t);
$nn='';

### now you're using <space><sep><space> as a separator
### I wanted <sep> only.  

for $i (@tokens){$nn.=$tag->{$i}." $sep ";}
my $z=rindex($nn,$sep);
my $len=length $sep;
substr($nn,$z,$len+1,'');
chop $nn;
$nn.='.mp3';

### the last 6 lines should've been something like:
### $nn = join($sep, map $tag->{$_}, @tokens) . ".mp3";
### and this is not my personal preferrence, but a more
### readable, straight to the point, honest to god statement

print "Renamed $_ to $nn \n" if rename $_,$nn; 
### where is my error check? 

}
print "\n\n";
print @errors;
[download]

And one more final thing which is more important than all of the above blabbering: do "perl name.pl '-' ARTIST" and wonder why all your artists suddenly have one song each.
Or worse: Watch all your mp3s reduced to a singleton ".mp3" as you forget to pass args other than the sep!!.

Hope this helps!!