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

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

##</code><code>##

exists argument is not a HASH or ARRAY element 

##</code><code>##

exists argument is neither a HASH element nor an ARRAY element 

##</code><code>##

if (exists $hash{some_key}){
#..
}

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