use strict;
our $aa=10;
our $vv=20;
my $x='asdf  <add>$aa + $vv</add> asdf  ';
$x =~ s/<add>(.*?)<\/add>/process($1)/gsei;
print $x;

sub process(){
my $var = shift;
print $aa;
print $vv;
print $var;    #here i need to variable interpolate the $var string
}