If your data file is close to the one below, this code should do it - notice that in the first line, I had some spaces, unlike all the following lines, to make sure it would do what I believe is the form your data is in.

Hope this helps 

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %seq_hash;
$/ = '>';
while ( <DATA> ) {
    if (s/^(\w+)\s\S+.*\n//) {
        chomp;
        tr/\n\t //d;
        $seq_hash{$1} = $_;
    }
}
print Dumper(\%seq_hash);

__DATA__
>YNL331C    sequence of the region upstream from ynl331c
CAATATG CGAG GGAC CTACATGTTGAGCATGACAATGAATTCTATTGAAAAAAAGAGTTG
GAAGTATATACGAATATAAATAATGTGAAACAAAAGAAGAAAAGTGAATAAAAGGCACTT
AAGACGCTATCCAATTGTGTATGAGAAGTGCAAACTCAATTTTTTTGCAAAAGACTTTTC
TCAAACCTTTAATTGATGCGTTCCGTTCATACATCAACCGCGTCCCATACGTTTCGAAGA
AAATTGCCTATATGTGTTATTTACATACTAGAGATATTTTAATATTGAAACAGTTGTATT
TCTATTGTAATTACTATCTAAACGTCTCGTCCCACCGCTGTTGATAAAGCGGTGCCAATA
TAATTTATAATCACGCCTGGGTAAATGACTTTTAATTTCTTAAATCATCGAAGTATGCGA
AAACAAGAAGTCTTTATTCATAATAAAAAACAAATTCGGTTACTACGACTTTTATATGTC
ATTCAATATTTGGTAATAATTTTACAGTATTAACCCTATCCTCATCTGATTCACTCTCTT
CTAATTGCATATATTTTCAAATTCGCTTTCAGCGCTGTACAAAACCAGTAAGCAGATCTC
GTACGAAGACTCAAATAAGTTGCATTGTTCGTATTCAAGGAAACCGGGGGGCAAAATTTC
CAACCATATTTAAGTATGACAATATTTCCAAGTCAAGGATGCATGCTGTTCTTCTCTTCA
TTAACTAGCTAACCAATTAGCTGAACGGCTTTGTATTTTACTTAACATATTGTCTATTGC
ATAAAAAACCACTATTCAGC
>YKL071W    sequence of the region upstream from ykl071w
ATAATTATTCCTGTTTCTTTAACCTGGTAAAAAAAAGTACAAACACTTAAGCTTTTTGAA
ACAGCTTTATTTTGCTTCATTAAATAGCTAGGATAAGAAATCCCTCATCCGAAAGGTTTT
GTATCTAACTACCCTAGAGAACATTTGTCCTGATCAGGTTCATTTGGAGTTTATATTTTT
TAGAAGCTCAAAGTTTGTTGGACTCATTACCATGGAAGAAAAAAAGAAGATACTACGAAA
TATTGGTTTCTCAGGTTAAATAAGGGACACCATTTTCCTATTAGGCTAGTCGAGCTTAGT
TCTTCTAATTTCTTCAGATCTTCTATAATTTCCTATCTTCTACCTGATGTGTGCATGATA
TATCTATGAGCTCCTGATATTGCTTGTTTTACTTTAGCTTGCATGACTTGCAATAATCTA
ATCATATATGTTCCCGATTAATATACTGTGCACAAATTGCAGGACATATAATTTTTCCGT
GGATTATATCTTCGATTAACGTCCGCGGGTCTCATAAAAAGCAAACCAACTTCGCAATTC
CCTAGAAATACCTCAATAGAAAGTTATTTGTAATGAGATTAGTAATGAGATTAGCAATGA
GATTAGTAATGAGATTAGTAATGAGATTAGTAATGTGATTAGTAATGCATAGCGGTATAA
ATGGTAGTACTAATAAGTAAGATAGTATACCAGTTATAATAAATAGGCGGCGATGCTTCA
AAACTAATTTTTGACGTTTTTAAGAATAAAGCCTTTACCAGTGGCATAAATCAGTAGAAT
TCTAAGCAAACAAAGTCGAT

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In reply to Re: Creating very long strings from text files (DNA sequences) by Anonymous Monk
in thread Creating very long strings from text files (DNA sequences) by bobychan

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