Here's an SSCCE:

#!/usr/bin/env perl 
use strict;
use warnings;
use Test::More tests => 1;

my $x = 7;
my $y = 5;
my $z;

my $formula = '$z = $x + $y';
eval $formula;
is ($z, 12)
[download]

Note that your script would be essentially executing arbitrary perl code from another source so this is a big security risk and you ought not to do it.