use warnings;
use strict;

my %barcode_hash = (
	1 => ['AGCTCGTTGTTCGATCCA','GAGAGATAGATGATAGTG','TTTT_CCCC',0],
	2 => ['AGCTCGTTGTTCGATCCA','GAGAGATAGATGATAGTG','TTTT_AAAA',0],
	3 => ['AGCTCGTTGTTCGATCCA','GAGAGATAGATGATAGTG','TTTT_BBBB',0],
	4 => ['AGCTCGTTGTTCGATCCA','GAGAGATAGATGATAGTG','TTTT_AAAA',0],
);
my $barcode_pair_35 = 'TTTT_AAAA';

for my $key (sort keys %barcode_hash) {
	print "Found at $key\n"
		if $barcode_hash{$key}[2] eq $barcode_pair_35;
}

# - OR -

my %lookup;
for my $key (sort keys %barcode_hash) {
	push @{ $lookup{ $barcode_hash{$key}[2] } }, $key;
}
print "Looked up at ".join(", ",@{$lookup{$barcode_pair_35}})."\n";

__END__

Found at 2
Found at 4
Looked up at 2, 4