use strict;
use warnings;

my %sym = ('x' => 4, 'y' => 7); 
while (<DATA>) {
    chomp; s/==/=/g; s/=/==/g;
    while (m/\$([a-zA-Z]\w*)/g) {
        $sym{$1} = 0 if ! exists $sym{$1};
    }
    s/\$([a-zA-Z]\w*)/\$sym{'$1'}/g;
    print "$_ evals as " . (eval() - 0) . "\n";
}

__DATA__
$x + $y = 11
$x + $y == 14
$x * ($x - $y) + 10
$x + $x1 + $x2