If those few operations are all you need, there's nothing wrong with the if/else dispatching above. However, if you ever want to extend it to other mathmatical operations, I'd use a dispatch table:

my %OPERATIONS = (
    '*' => sub { $_[0] * $_[1] }, 
    '-' => sub { $_[0] - $_[1] }, 
    '+' => sub { $_[0] + $_[1] }, 
);
# $operator, $first, and $second defined 
# elsewhere, from user input
print "Result: ", 
    $OPERATIONS{$operator}->($first, $second), 
    "\n"
    if exists $OPERATIONS{$operator};
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Also, avoid using $a and $b as variable names, as they have special meaning in sort subroutines.