It seems to me that it's impossible to use /e to solve this situation when variable is lexical scoped.

There's no way around that using symbolic variables because symbolic variables only access package variables. The easiest drop in would be using a hash, which might look like:

my %hash = (AGE => 17,
            );
$text =~ s/\$(\w+)/$hash{$1}/g;
[download]

This has the advantage of removing a bunch of misdirection and potential security complications. If you wanted to make key misses fatal, you can use lock_hash in Hash::Util; of course, your symbolic reference code doesn't die on misses.

Yes, sprintf will work for this case as we know it's a number; but in other cases we may not know if $AGE holds a numeric value.

sprintf works for any string. Note I use %s for string replacement; numerical replacement would be %d. Obviously the multiplication only works if $AGE is a number, but that's sort of given.

If you are learning this with an eye toward production code, please read Why it's stupid to use a variable as a variable name. Also, there are a large number of real templating packages out there, such as HTML::Template for simple projects and Template::Toolkit for more complex ones.