use Data::Dumper;
# Input data
my %data = ( 1 => [qw/I don't know/], 2 => [qw/what to use/], 3 => [qw/as values !/] );

# Repeat multiple times:
#  A signed 32 bit integer followed by three null terminated strings
my $pattern = "(l(Z*)3)*";

# Encoding
my $packed = pack $pattern, map { $_, @{$data{$_}} } keys %data;

# Decoding
my @unpacked = unpack $pattern, $packed;
my ($key, @values, %out);
$out{$key} = [@values] while (($key, @values) = splice @unpacked, 0, 4);
print Dumper \%out;