#!/usr/bin/perl
use strict;
use warnings;

my @out = ( 1, undef, 3 );
my @in  = ( 4, 5, undef );

OUTER: 
for my $o ( @out ) {
  INNER: 
  for my $i ( @in ) {
    if ( !defined $o ) {
      next OUTER;
    }
    
    if ( !defined $i ) {
      next INNER;
    }

    printf "\$o(%d) * \$i(%d) = %d\n", $o, $i, $o * $i;
  }
}