./rename *.pl "s/^/old_/"

# will be expanded to this:

./rename 1.pl 2.pl 3.pl s/^/old_/

# and set @ARGV like so:
$ARGV[0] == '1.pl';
$ARGV[1] == '2.pl';
$ARGV[2] == '3.pl';
$ARGV[3] == 's/^/old_/';

##</code><code>##

my $pattern = pop @ARGV;
for my $file (@ARGV){
   my $new = $file;
   # apply pattern and rename
}