my( $z, $zEnd, $i, $iEnd )= unpack "z.I.", $buf;
# $zEnd == length($z)
# $iEnd == 4 + $zEnd

my( $z, $i )= getData();
my $buf= pack "z.I.", $z, my $zEnd, $i, my $iEnd;
# $zEnd == length($z)
# $iEnd == 4 + $zEnd