BCD, Binary Coded Decimal is a system where each 4 binary bits represent a decimal digit. The hexadecimal digits AF are not used in this representation.
This format is a bit odd nowadays (more normal would be ASCII decimal which takes 2 8 bit bytes instead of 2 4 bit nibbles, but that extra space is usually not of consequence). Anyway this is how to calculate the "digits". The integer "div" or modulo operation is only one machine instruction and the integer multiply is also only one machine instruction. I don't know how good Perl is at this. See Perl pack() and unpack() for how to cram these into one 8 bit value.
#!usr/bin/perl w
use strict;
my @tests = (2,11,23,89,99);
foreach my $num (@tests)
{
my $units = $num % 10; #modulo is the remainder
my $tens = ($num  $units)/10;
printf "decimal=%02d BCD=%04b %04b\n", $num, $tens, $units;
}
__END__
decimal=02 BCD=0000 0010
decimal=11 BCD=0001 0001
decimal=23 BCD=0010 0011
decimal=89 BCD=1000 1001
decimal=99 BCD=1001 1001
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