Re^3: why lexical variables can not be interpolated?

[kennethk]

As described in s/PATTERN/REPLACEMENT/msixpodualgcer in perlop,

e Evaluate the right side as an expression.
ee Evaluate the right side as a string then eval the result.

The reason $text =~ s/(\$\w+)/$1/eeg; works is the regular expression stores $AGE (string literal) in $1. For the substitution, $1 is evaluated to return $AGE, and then $AGE is evaluated to return 17. Your second code attempts to multiply the string literal $AGE by 2, not the value of it - you've got your order of operations off. You could accomplish this using an explicit dereference rather than invoking eval twice, like

$text =~ s/\$(\w+)/$$1 * 2/egx;  #I tried to double the age and succee
+ded!

What I've changed:

1. I used an explicit dereference $$1, which could also be expressed as ${$1}
2. I changed the pattern so that the sigil is replaced, but is not part of the pattern. This is necessary because $$1 attempts to access the scalar variable named AGE; otherwise it would assume you are looking for a variable with an explicit dollar sign in its name.
3. I changed the code to eval only once.

I realize that this is a learning exercise, but for reference if I were going to do this kind of templating, I would use sprintf in something like:

$AGE = 17;
$tmpl = 'I am %s years old';      # note single quotes
$text = sprintf $tmpl, $AGE * 2;  # print I am 17 years old