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+-------------+ +------------------+ 
| Products    | | Supplier_Product | 
...
| P1  | Screw | | S1      | P2     | 
| P2  | Screw | +------------------+ 
+-------------+

SELECT parts.pno 
FROM   parts
...
      (SELECT supplier_parts.pno
       FROM   supplier_parts
       WHERE  supplier_parts.sno = 'S1')

SELECT DISTINCT parts.pno 
FROM   parts
...
      (SELECT supplier_parts.pno
       FROM   supplier_parts
       WHERE  supplier_parts.sno = 'S1')

use warnings;

my $foo = undef;
print "This is some ", $foo, "\n";

perl somescript.pl 1 2 three four

my $sum;
$sum += $_ foreach @ARGV;
print $sum, "\n";