perldoc -q "expand variables" says:

       How can I expand variables in text strings?

       Let's assume that you have a string like:

           $text = 'this has a $foo in it and a $bar';

       If those were both global variables, then this would suffice:

           $text =~ s/\$(\w+)/${$1}/g;  # no /e needed

       But since they are probably lexicals, or at least, they could b
+e, you'd
       have to do this:

           $text =~ s/(\$\w+)/$1/eeg;
           die if $@;                  # needed /ee, not /e

       It's probably better in the general case to treat those variabl
+es as
       entries in some special hash.  For example:

           %user_defs = (
               foo  => 23,
               bar  => 19,
           );
           $text =~ s/\$(\w+)/$user_defs{$1}/g;
[download]