my %a=('la'=>1, 'ka'=>5, 'pika'=>4);
my %b=('pika'=>5, 'kasa'=>4, 'ka'=>44);

my %common_keys=();

foreach my $key (keys %a)
{
if(exists $b{$key})
    {
$common_keys{$key}='yes';    
    }
}

my $the_common_keys_str=join(",", keys %common_keys);
print "The common keys are: " . $the_common_keys_str;