As noted above, MD5 is a one-way function, and you need a two-way function for this purpose.

After some false starts, I realized that reversing the number, so that the least significant digit was on the left, does a good job of making the encoded versions non-sequential.

A bit more fiddling produced the below; fiddle with the three key_* constants to produce your customized local encoding pattern.

use strict;
use Math::BaseCalc;

### Change the three lines below to ensure your coding is unique

my $key_bitxor = 1231456789;
my $key_multip = 3;
my @key_digits = ( 'N' .. 'Z', 2 .. 9, 'a' .. 'm', 'A' .. 'M', 'n' .. 
+'z' );

### Coding and decoding functions

my $bcalc = Math::BaseCalc->new( digits => \@key_digits );

sub numeric_obfu {
  my $number = shift;
  my $flipped = ( $number * $key_multip )  ^ $key_bitxor;
  $flipped =~ s/^(\d+?)(0*?)$/reverse($1).$2/e;
  return $flipped;
}

sub numeric_defu {
  my $result = shift;
  $result =~ s/(\d+?)(0*?)$/reverse($1).$2/e;
  $result = ( $result ^ $key_bitxor ) / $key_multip;
  return $result;
}

### Print sample cases

my @tests = ( 0..10, 998..1004, 9999995..10000004, 99999990..99999999 
+);
foreach my $sample ( @tests ) {  
  my $obfu = numeric_obfu( $sample );
  my $code = $bcalc->to_base( $obfu );
  my $dcod = $bcalc->from_base( $code );
  my $defu = numeric_defu( $dcod );
  print "Converted $sample : $obfu : $code : $dcod : $defu : " . 
    ( $defu == $sample ? 'OK' : $dcod) . "\n";
}
[download]

Sample output:

Converted 0 : 9876541321 : ZIRJIO : 9876541321 : 0 : OK
Converted 1 : 9765413210 : Zmj3Mq : 9765413210 : 1 : OK
Converted 2 : 7876541321 : XULhVH : 7876541321 : 2 : OK
Converted 3 : 6976541321 : Vy7oVH : 6976541321 : 3 : OK
Converted 4 : 3976541321 : STpuoH : 3976541321 : 4 : OK
Converted 5 : 4976541321 : TczlBa : 4976541321 : 5 : OK
Converted 6 : 5776541321 : UeJ3oH : 5776541321 : 6 : OK
Converted 7 : 8676541321 : YWiXbO : 8676541321 : 7 : OK
Converted 8 : 1876541321 : PdnHhH : 1876541321 : 8 : OK
Converted 9 : 2876541321 : QHx84a : 2876541321 : 9 : OK
Converted 10 : 9776541321 : ZAaLva : 9776541321 : 10 : OK
Converted 998 : 1364541321 : OL684a : 1364541321 : 998 : OK
Converted 999 : 4264541321 : SiQ3oH : 4264541321 : 999 : OK
Converted 1000 : 7364541321 : Wh4Sva : 7364541321 : 1000 : OK
Converted 1001 : 8364541321 : XLdJIO : 8364541321 : 1001 : OK
Converted 1002 : 5364541321 : TtvqbO : 5364541321 : 1002 : OK
Converted 1003 : 6764541321 : VHx84a : 6764541321 : 1003 : OK
Converted 1004 : 3764541321 : Rqheva : 3764541321 : 1004 : OK
Converted 9999995 : 4174491210 : SbT9tj : 4174491210 : 9999995 : OK
Converted 9999996 : 7317449121 : WdDRLa : 7317449121 : 9999996 : OK
Converted 9999997 : 8317449121 : XHMIlO : 8317449121 : 9999997 : OK
Converted 9999998 : 1517449121 : OxS2EH : 1517449121 : 9999998 : OK
Converted 9999999 : 4417449121 : SGrWZO : 4417449121 : 9999999 : OK
Converted 10000000 : 9817449121 : ZDkWZO : 9817449121 : 10000000 : OK
Converted 10000001 : 9174491210 : YnuiMq : 9174491210 : 10000001 : OK
Converted 10000002 : 7817449121 : XQYtEH : 7817449121 : 10000002 : OK
Converted 10000003 : 6917449121 : VtL2EH : 6917449121 : 10000003 : OK
Converted 10000004 : 3917449121 : SP597H : 3917449121 : 10000004 : OK
Converted 99999990 : 5582525841 : UXLODa : 5582525841 : 99999990 : OK
Converted 99999991 : 8482525841 : XujxXH : 8482525841 : 99999991 : OK
Converted 99999992 : 1682525841 : PWph6a : 1682525841 : 99999992 : OK
Converted 99999993 : 2682525841 : QfzTRO : 2682525841 : 99999993 : OK
Converted 99999994 : 9582525841 : Z8clKO : 9582525841 : 99999994 : OK
Converted 99999995 : 6382525841 : VZhJqH : 6382525841 : 99999995 : OK
Converted 99999996 : 3382525841 : R9zqjH : 3382525841 : 99999996 : OK
Converted 99999997 : 4382525841 : SEWh6a : 4382525841 : 99999997 : OK
Converted 99999998 : 7482525841 : WDa8dO : 7482525841 : 99999998 : OK
Converted 99999999 : 4825258410 : TZ8Wmj : 4825258410 : 99999999 : OK
[download]

Obviously, that's just obfuscated, not really encrypted, and if someone assembled a list of codes in incremental numeric sequence they'd be able to figure things out, but it's pretty opaque to your typical end-user.