Efficently?
Not sure ;)

my %h = 
map{ reverse /(\d+).*#(.+)/ } grep "@array1", @array2;
print Dumper \%h;
[download]

Output:
$VAR1 = {
          'def' => '890',
          'abc' => '15',
          'ghi' => '78'
        };
[download]

hth,
PooLpi