I presume you mean like a view-source option for the script itself? Well, if it is just for scripts you could do something like:

use CGI;
my $q=new CGI;
print $q->header("application/x-perl").`cat $0`;
[download]

Whereas if you're talking about *any* file (eg image files etc) you need to do a bit of juggling with mime types, and of course there's the whole attachment/filename issue also. Something like this would do for that:

use CGI;
my $q=new CGI;
my $filename=$q->param("some.param.name");
die "I won't display the file: $filename" unless some_test_or_other($f
+ilename);
print $q->header(-type=>`file -bi $filename`."; file=$filename",
   -content_dispostion=>"attachment; filename='$filename'").
   `cat $filename`;
[download]

WARNING: Be very very careful if you do something like this. File-download scripts are responsible for a great many web site security holes, so unless you intend to impliment really anal input checking, don't even consider doing a (named) file download script as described above.

Also please note that using - and trusting - 'cat' and 'file' to do work for you is both inefficient and potentially insecure. They're included in these examples for brevity, in any real environment you should generally use a self-written function or CPAN module as appropriate.