In Perl, there are 2 different kinds of hashes.

• A hash, which is defined by the % and ( ) characters - %hash = ( );
• A reference hash, which is defined with the $ and { } characters - $hash = { };

Let's say you create an empty reference hash.

my $hash = {};

Dumper can be used to display the structure of the hash.

use Data::Dumper;
print Dumper \%hash;

Which will return the following.

$VAR1 = {};

The following if statement will return "is defined".

if ( $hash ) {
  print "The reference hash named \$hash is defined\n";
}

The following will return "does not contain the foo key" since the reference hash does not contain any keys (or values).

AVOID TROUBLE

Do not wrap the hash reference in double quotes, such as "$hash->{foo}", as this can cause the evaluation to fail.

if ( exists $hash->{foo} ) {
  print "The reference hash named \$hash contains the 'foo' key \n";
}
else {
  print "The reference hash named \$hash does NOT contains the 'foo' key \n";
}

Let's create an empty key.

$hash->{foo} = "";

Dumper will return the following.

$VAR1 = {
           'foo' => ''
        };

Now the following will return "contains the foo key" since the reference hash does contain the foo key.

if ( exists $hash->{foo} ) {
  print "The reference hash named \$hash contains the 'foo' key \n";
}
else {
  print "The reference hash named \$hash does NOT contains the 'foo' key \n";
}

The following will return "is empty (contains no value)".

if ( $hash->{foo} ) {
  print "The \$hash->{foo} is not empty \n";
}
else {
  print "The \$hash->{foo} key is empty (contains no value) \n";
}

Let's define a string element.

$hash->{foo} = "bar";

Dumper will return the following.

$VAR1 = {
           'foo' => 'bar'
        };

The following if statement will return "is not empty".

if ( $hash->{foo} ) {
  print "The \$hash->{foo} is not empty \n";
}
else {
  print "The \$hash->{foo} key is empty (contains no value) \n";
}

Let's define a boolean element.

$hash->{foo} = 0;

The following if statement will return "is empty", meaning that boolean 0 is interpreted as empty.

if ( $hash->{foo} ) {
  print "The \$hash->{foo} is not empty \n";
}
else {
  print "The \$hash->{foo} key is empty (contains no value) \n";
}

Let's define a boolean element greater than 0.

$hash->{foo} = 1;

The following if statement will return "is not empty", meaning that a boolean greater than 0 is interpreted as not empty.

if ( $hash->{foo} ) {
  print "The \$hash->{foo} is not empty \n";
}
else {
  print "The \$hash->{foo} key is empty (contains no value) \n";
}