Just to add a little bit for the OP to understand:

my %hash = ("a" => undef, "b" => 0, "c" => 2);

testing("a");
testing("b");
testing("c");
testing("d");

sub testing {
    my $key = shift;
    print "$key exists\n" if (exists $hash{$key});
    print "the value of $key is defined\n" if (defined $hash{$key});
    print "the value for $key is true" if ($hash{$key});
}
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This prints:

a exists
b exists
the value of b is defined
c exists
the value of c is defined
the value for c is true
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