You would slurp the whole file in a scalar variable (there are two ways to do this: ($file) = <FH>; or undef $/; $file = <FH>;). Then you just follow srawl's advice and split it on '::':

@things = split/::/$file;
[download]

Then you have in $things[0] the HTML to be edited, and the other things it will split on.

Tiptoeing up to a Perl hacker.
Dave AKA damian