if ($x == "0"){ $num_c = $num_a + $num_b }
if ($x == "1"){ $num_c = $num_a - $num_b }
if ($x == "2"){ $num_c = $num_a * $num_b }

##</code><code>##

# ...
my $equat = "$num_a $symbols[$x] $num_b";
print "$equat = ";
# ...
$num_c = eval $equat;
# ...