I'd imagine the map is the only part that is really confusing; here it is in a nutshell:

@names = map ( # start map

/ # start matching regex

# this will place what is matched in the grouping
# into $_, or undef if there are no matches

\A           # match start, kinda like ^,
             # except will ONLY match begining
\/  ?        # 0 or 1 literal backslashes
\Q           # start regex quotemeta 
$path[$step] # interpolated, and then quotemeta'd
\E           # end regex quotemeta
(?:          # non grouping parenthesis
\/           # literal backslash
(.*)         # here is what actually is grouped; 
             # matches the rest of the line
)?           # end non-grouping parens, 0 or 1 of those
\z           # match end, kinda like $,
             # except will ONLY match very end of string
/sx          # s lets . match newlines; i added the x
, @names);   # i added parenthesis
[download]