my %hash = (    Key1 => 'Pigeon.Lion.Tiger.Elephant',
                Key2 => 'Pigeon.Lion.Tiger.Rabbit',
                Key3 => 'Pigeon.Lion.Tiger.Monkey',
);
my %lookup = reverse %hash;

while (<DATA>)  {
    chomp;
    my ($value) = split;
    my $key = $lookup{$value};
    print "$_ ($key)\n" if /SOME_DATA/ and defined $key;
}

__DATA__
Pigeon.Lion.Tiger.Elephant SOME_DATA
Pigeon.Lion.Tiger.Lion SOME_DATA
Pigeon.Lion.Tiger.Monkey SOME_DATA

##</code><code>##

Pigeon.Lion.Tiger.Elephant SOME_DATA (Key1)
Pigeon.Lion.Tiger.Monkey SOME_DATA (Key3)