my @array = qw(world today is very nice);
my %hash = (0 => "Hello", 1=> "world" , 2=> "today" , 3=> "is" , 4=> "nice" );

my %reversed_hash = reverse %hash;
foreach my $word ( @array )
{
    if ( exists $reversed_hash{ $word } )
    {
        print $reversed_hash{ $word } . "\n";
    }
    else
    {
        print "unknown '$word'\n";
    }
}