Since you are a "totally Perl illiterate", you might like a description of what the code actually does.

1: my($thebase) = ($file =~ /(.*)\/[^\/]+/);
2: my($html,$html2);
3: if (!-e $file){
4: $thebase =~ s/\/[^\/]+$/\/default/;
[download]

as an example, I set $file = "1/2/3/4.txt";
1: applies a regular expression to $file; assigns the directory $file resides in to $thebase ($thebase = "1/2/3").
3: checks whether $file does not exist.
4: if $file does not exist, the last directory component of $thebase is replaced by "default". ($thebase = "1/2/default").

daniel.