$ perl -e '@ARGV ||= "."'
Can't modify array dereference in logical or assignment (||=) at -e li
+ne 1, at EOF
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Don't get your contexts mixed up. The left side of a logical operator is always in scalar context. Also don't forget - rename may fail, so if you're going to return the new filename you should be sure the renaming succeeded. Overall this seems more like a job for map:

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