First, your @array_num variable is getting populated with a single value the way that your code is written. Try this:

my @array_num = qw/123 456 345 459 234/;
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Second, you never set $n1 and $n2 to anything. I'll assume that you want them to be the first and second values in your array.

my $n1 = $array_num[0];
my $n2 = $array_num[1];
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Third, you really should be using placeholders (or bind parameters) in your $sql statement to safeguard against SQL injection attacks. This is not much of a problem in the statements as you've presented them, but it's really a good practice to get into. Read more about SQL placeholders.

So, finally, we have this:

my @array_num = qw/123 456 345 459 234/;
my $n1        = $array_num[0];
my $n2        = $array_num[1];

my $sql= "
  SELECT tb.num1, tb.num2 
  FROM   MYTABLE tb 
  WHERE  tb.num1 = ?
  AND    tb.num2 = ?";

$sql->execute($n1, $n2);
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EDIT: Per trammell's reply below.