Anonymous Monk,
You could get around your problem if you re-write your code as follows:

for (my $i = 0; $i < $#array; $i++) {
    if ($array[$i] =~ m/\D{1}(\D?\d+\D?\d*)\D{1}(\D?\d+\D?\d*)\D{1}/ )
+ {
        $x1 = $2;
        $y1 = $1;
    }
    if (($array[$i + 1] =~ m/\D{1}(\D?\d+\D?\d*)\D{1}(\D?\d+\D?\d*)\s*
+\D{1}/ ) {
        $x2 = $2;
        $y2 = $1;
    }
}
[download]

This is because $#array will be the index of the last element. By stopping at one less than this value, you know that $i + 1 will still be defined. This is not the best code, but I worked with what you had already shown - good job.

Cheers - L~R