Wouldn't this work? I haven't even run this to check for syntax, so DON'T EVEN trust me! I'm just as clueless as you are even more so probably, but I would like to know if this is generaly possible && if there are any downs of using this kind of approach.

print &FilesExt('/tmp','csv','txt')." files renamed\n";

sub FilesExt {

    my ($startpath,$orgext,$newext) = @_;
    my ($count);

    foreach $leaf (glob("$startpath/*")) {
        if (-d $leaf) { 
            $count += FilesExt($leaf,$orgext,$newext);
        } elsif ($leaf =~ m/$orgext\z/i) { #case insensitive maybe?
            my $oldleaf = $leaf;
            $leaf =~ s/$1/$newext/;
            rename($oldleaf,$leaf) or warn();
            ++$count;
        }
    }
    return($count);
}

Comments, monks?