$ perl -e '@ARGV ||= "."'
Can't modify array dereference in logical or assignment (||=) at -e line 1, at EOF

##</code><code>##

sub pp {
    map {
        my $new = lc $_;
        ($new eq $_ or -e $new)
            ? $_
            : rename($_, $new) ? $new : $_;
    } @_;
}