Perhaps you might save us the trouble and let Perl explain the problem to you:

# like all good Perl programmers [insert name here] always
# tests the return value of functions like open, unlink, 
# rename etc and includes the error message kindly put by 
# Perl into $! so the reason for a failure is made 
# immediately obvious in all its gory diagnostic detail.....

rename $old, $new or die "Can't rename Perl say reason $!\n";
[download]

Permission or path if you want the short answer. $! if you want the exact details.