use strict;
use warnings;
use Data::Dump;

my $header;
my %sequence;

while ( my $line = <DATA> ){
    chomp $line; 
    if ( $line =~ /^>(.*)/ ) {
        $header = $1;
    } else {
        $sequence{$header} .= $line;
    }
}

dd \%sequence;

	
    
__DATA__
>sequence_5849
CCTGCGGAAGATCGGCACTAGAATAGCCAGAACCGTTTCTCTGAGGCTTCCGGCCTTCCC
TCCCACTAATAATTCTGAGG
>sequence_5959
CCATCGGTAGCGCATCCTTAGTCCAATTAAGTCCCTATCCAGGCGCTCCGCCGAAGGTCT
ATATCCATTTGTCAGCAGACACGC
>sequence_0808
CCACCCTCGTGGTATGGCTAGGCATTCAGGAACCGGAGAACGCTTCAGACCAGCCCGGAC
TGGGAACCTGCGGGCAGTAGGTGGAAT

##</code><code>##

{
  sequence_0808 => "CCACCCTCGTGGTATGGCTAGGCATTCAGGAACCGGAGAACGCTTCAGACCAGCCCGGACTGGGAACCTGCGGGCAGTAGGTGGAAT",
  sequence_5849 => "CCTGCGGAAGATCGGCACTAGAATAGCCAGAACCGTTTCTCTGAGGCTTCCGGCCTTCCCTCCCACTAATAATTCTGAGG",
  sequence_5959 => "CCATCGGTAGCGCATCCTTAGTCCAATTAAGTCCCTATCCAGGCGCTCCGCCGAAGGTCTATATCCATTTGTCAGCAGACACGC",
}

##</code><code>##

    if ( $line =~ /^(HEAD_PATTERN)/ ) {
        $data    = \ $deeply{nested}{structure}{$1}; # reference data        
    } else {
        $$data .= $line;                            # derefrence data
    }