my ($k,$v, %seq);
for my $line (<DATA>) {
  chomp ($line);
  if ( $line=~/^>(\S+)\s*(.*)/){
     $k=$1;
     $v=$2;
  }else{
     $v=$line;
  }
  $seq{$k} .= $v; 
}
#---- Print it ----
for my $k(sort keys %seq){
  print "$k \t=> $seq{$k}\n";
}
# --- Output ---(Using the __DATA__ block above) ..-
#  123 	=> blahabcdefghijkl
#  456 	=> de dahmnopqr
#  789 	=> nothing wanted herestuvwxyz