You could use a dispatch table.

For example:

my $pdispatch = {
    '+' => sub { $_[0] + $_[1] },
    '-' => sub { $_[0] - $_[1] },
    '*' => sub { $_[0] * $_[1] },
    '/' => sub { $_[0] / $_[1] },
};

# and later ...
my $operator = qw(+ - * /)[int rand 4];
my $true_answer = $pdispatch->{$operator}->($value1, $value2);
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The dispatch table $pdispatch is a hash reference, where each key evaluates to an anonymous subroutine to perform that operation on its two arguments.

The operator $operator is randomly selected from a list of 4 operators, and $true_answer is assigned the results of the subroutine for the random operator, performed on its 2 arguments.