The problem is that the code is potentially ambigous.

There are two ways to parse it:

s{...}{
 # much stuff here
}x
[download]

The other, but less obvious, ist this:

s{...}{ 
# some characters 
{}{
# more characters...
}
# no terminator
[download]

When perl tries to parses the regex it doesn't know if there is going to come an /x modifier, so the braces have to be balanced - even in comments.

That's one of the reasons Perl 6 puts the modifiers at the start of the regexes.