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;