hi=3 ; 
my=4; 
before substitution. 
hi + my 
afetr substitution  
3 +4. 
now i want  
7

use strict; 
use warnings; 
my $string = 'hi + my'; 
my $hi = 3; 
my $my = 4; 
print $string, "\n"; 
$string =~ s/hi/$hi/; 
$string =~ s/my/$my/; 
print $string, "\n"; 
my $sum = eval($string); 
print $sum, "\n";