examples/delete.pl

#!/usr/bin/perl 
use strict;
use warnings;
use Data::Dumper qw(Dumper);

my %phones = (
    Foo => '111',
    Bar => '222',
    Moo => undef,
);

print exists $phones{Foo}  ? "Foo exists\n"        : "Foo does not exist\n";
print Dumper \%phones;


delete $phones{Foo};

print exists $phones{Foo}  ? "Foo exists\n"        : "Foo does not exist\n";
print Dumper \%phones;

At first the key "Foo" exists in the hash and after calling delete it does not exist any more.

Result:

Foo exists
$VAR1 = {
          'Foo' => '111',
          'Moo' => undef,
          'Bar' => '222'
        };
Foo does not exist
$VAR1 = {
          'Moo' => undef,
          'Bar' => '222'
        };