$x = 0;
$y = 0;
$oper = "";
print "a=\n";

$x = <>;
chomp ($x);
print "operation +, -, *, /:\n";

$oper = <>;
chomp ($oper);
print "b=\n";

$y = <>;
chomp ($y);
if ($oper eq "+") {$result = $x + $y;}
elsif ($oper eq "-") {$result = $x - $y;}
elsif ($oper eq "*") {$result = $x * $y;}
elsif ($oper eq "/") {$result = $x / $y;}
print "$result";
<>;

print "a=\n";
$a = <>;

print "operation:\n";
$oper = <>;

print "b=\n";
$b = <>;

$c = $a [и тут как нибудь вставить переменную $oper] $b;

print "$c";

...
$c = $a.$oper.$b;
print "$c";