You've chosen an effective use for a hash, but if you only need to find the longest (or longer or only) sequence for an ID that has one or more sequences, consider the following solution that doesn't use an array:

use strict;
use warnings;

my %FASTAhash;

open my $file, '<FASTA.txt' or die $!;
while (<$file>) {
    next if !/(>[^ ]+) /;
    chomp( $FASTAhash{$1} = $' )
      if !$FASTAhash{$1}
          or length $' > length $FASTAhash{$1};
}
close $file;

print "$_ $FASTAhash{$_}\n" for keys %FASTAhash;

[download]

The regex matches the ID, which is placed into $1, leaving the remaining (unmatched) sequence in $'. The hash item whose key is the ID in $1 is assigned the sequence in $' and then chomped if that item's undefined (in this case) or the length of $' is greater than what's already there. When done, each ID is paired with its longest sequence. (Is it possible for two sequences of the same ID to be the same length? If so, do you need to code for that?)

Output from processing your data:

>ENSG00000147724 MSEIQGTVEFSVELHKFYNVDLFQRGYYQIRVTLKVSSRIPHRLSASIAGQTE
+SSSLHSA CVHDSTVHSRVFQILYRNEEVPINDAVVFRVHLLLGGERMEDALSEVDFQLKVDLHFTDS 
+EQQLRDVAGAPMVSSRTLGLHFHPRNGLHHQVP
>ENSG00000010072 MDDDLMLALRLQEEWNLQEAERDHAQESLSLVDASWELVDPTPDLQALFVQFN
+DQFFWGQ LEAVEVKWSVRMTLCAGICSYEGKGGMCSIRLSEPLLKLRPRKDLVETLLHEMIHAYLFV 
+TNNDKDREGHGPEFCKHMHRINSLTGANITVYHTFHDEVDEYRRHWWRCNGPCQHRPPYY GYVKRATN
+REPSAHDYWWAEHQKTCGGTYIKIKEPENYSKKGKGKAKLGKEPVLAAENKD KPNRGEAQLVIPFSGK
+GYVLGETSNLPSPGKLITSHAINKTQDLLNQNHSANAVRPNSKI KVKFEQNGSSKNSHLVSPAVSNSH
+QNVLSNYFPRVSFANQKAFRGVNGSPRISVTVGNIP KNSVSSSSQRRVSSSKISLRNSSKVTESASVM
+PSQDVSGSEDTFPNKRPRLEDKTVFDNF FIKKEQIKSSGNDPKYSTTTAQNSSSSSSQSKMVNCPVCQ
+NEVLESQINEHLDWCLEGDS IKVKSEESL*
>ENSG00000067082 Sequence unavailable

[download]

Hope this helps!

In reply to Re: Saving different values for the same key by using Hash of Arrays by Kenosis
in thread Saving different values for the same key by using Hash of Arrays by beginner27

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