use Data::Dumper;
my %hash    = (
 'mus09r' => [ '0,-464,AAACCATCTTGAAAC',
               '0,-350,GGTTCAGGATGGTTT',
               '0,-75,AAAACATCGTGACAC' ],

)


while (my ($file,$val) = each %hash) {
      print '>dataset\n";
      print '$file\n";
      print '>instances\n";
      print join("\n",@{$val}),"\n";
}

##</code><code>##

$VAR1 = 
'
>dataset 
mus09r
>instances
0,-464,AAACCATCTTGAAAC
0,-350,GGTTCAGGATGGTTT
0,-75,AAAACATCGTGACAC
'