A quick idea of what you can do:

print <<END;
// JavaScript goes here

// ...in large quantities.
END
# Perl continues here
[download]

The last part can be changed to 'END' if you want to prevent variables such as $foo from being interpolated. Of course, you might want this, so the option is there.

Another way to do this is using standard quotation:

print qq~
// JavaScript goes here

// ...as long as there's no tildes (which end it)
~;
# Perl continues here
[download]

As a tip, qq interpolates like double quotes, while q alone is like a single quoted string. Carefully choose your delimiter, though, since it can confuse the parser if it appears within your JavaScript. Tilde is a fairly safe bet, I would presume.