Select Code to Download

for my $key (keys %hash){
#...
...
for my $key (keys %{$hashref}){ # dereferencing a hash
#...
}

exists argument is not a HASH or ARRAY element

exists argument is neither a HASH element nor an ARRAY element

if (exists $hash{some_key}){
#..
...
if (exists $hashref->{some_key}){ # dereferencing a hash element using
+ an arrow
#..
}