joakim@debian:~/perl$ ls
BBB.txt  AAA.txt
joakim@debian:~/perl$ perl -we 'opendir my $dh, "." or die "Unable to open dir . : $!\n"; while (defined(my $line = readdir $dh)) { unless ($line =~ /^\.$|^\.\.$/) { rename $line, lc $line or die "Unable to rename $line: $!\n"; print "Renamed $line\n"; } }'
Renamed AAA.txt
Renamed BBB.txt
joakim@debian:~/perl$ ls
bbb.txt  aaa.txt