use strict;
my @a = qw( 1 2 3 );
my @b = qw( 7 8 9 10 );
my ($x, $y) = (1, 2);

for (my $i = 0; $i < @a || $i < @b;  ) {
    printf( "$x: %s $y: %s\n", 
            map { defined $_ ? $_ : 'undef' } ( $a[$i], $b[$i++] ) );
}