Hi, thanks, that was almost exactly it. This does what I wanted:

my $zip = Archive::Zip->new( 'browser.jar' ) || die "$!";

# extract what you want
my $file = "content/browser/browser.js";
$zip->extractMember($file);

# Edit the file as necessary,
# which is at ./content/browser/browser.js

# now add it back
$zip->removeMember($file);
$zip->addFile($file);
$zip->overwrite();
[download]

The extractMember function actually does extract a tree, as in, it extracts whatever directories are necessary to mirror that one file and its enclosing directories. So the file is there in the current working directory, inside the portion of the tree where it was located in the zip file.