use strict;

my @array = (13, 27, 4, 18, 12, 6);
my %hash;
@hash{@array} = (0..$#array);
my $index = $hash{18};

print "should be 18: ", $array[$index], "\n";
print "should be 4: ", $array[$index-1], "\n";
print "should be 12: ", $array[$index+1], "\n";