my $hash = 
{ 
    a1 => 
    { 
        a2 => 'Hi' 
    } 
};

my @key = qw {a1 a2};

print "Hash-value is ", lookup($hash, @key), "\n";

sub lookup
{
    my $hash = shift;
    my $key  = shift;
    
    return @_ ? lookup($hash->{$key}, @_) : $hash->{$key};
}