chomp(my $left = <STDIN>);
chomp(my $op = <STDIN>);
chomp(my $right = <STDIN>);

print $op eq '+' ? $left + $right :
      $op eq '-' ? $left - $right :
      $op eq '*' ? $left * $right :
      $op eq '/' ? $left / ($right||1) :
      "dunno operator '$op'",
, "\n";

##</code><code>##

chomp( my $expr = <STDIN> );
# and to parse it with simple split
my ($left,$op,$right) = split /\s+/,$expr
# next step is code of above

##</code><code>##

my %operations = (
  '+' => sub { (shift) + (shift) },
  '-' => sub { (shift) - (shift) },
  '*' => sub { (shift) * (shift) },
  '/' => sub { (shift) / (shift||1) },
);

my ($left,$op,$right) = ...; #wherever you get them from
print exists $operations{$op} ?
    $operations{$op}->($left,$right) :
    "dunno operator '$op'",
"\n";

##</code><code>##

chomp(my $expr = <STDIN>);
$expr =~ /(\d+)\s+([+-/*])\s+(\d+)/ or
   die "invalid input\n";
my ($left,$op,$right) = ($1,$2,$3);
# ...