my $msf0_fmt = "c3x"; # x because this is likely to 
                      # be padded. omit as needed
my $cdrom_addr_fmt = "$msf0_fmt I";
my $cdrom_tocentry_fmt = "c4 $cdrom_addr_fmt c"; # will need padding
                                                 # if struct cdte_addr is larger
my $str = pack $cdrom_toentry_fmt; # make the string long enough
# ...
# call the ioctl on $str
my @data = unpack $cdrom_toentry_fmt, $str;