As others have pointed out, your description of the problem is a little difficult to understand. However, this code produces the same results as the examples you give so perhaps it is what you want :)

use strict;
use warnings;

my @tests = (
   {
      str => q{423RY75Y69827EC67592C78657N965R},
      ans => q{423R_Y75Y_69827E_C67592C_78657N965R}
   },
   {
      str => q{423RY75Y69827EC67592C78657N965R345U299M},
      ans => q{423R_Y75Y_69827E_C67592C_78657N965R_345U_299M}
   });

my $rxDigit    = qr{[N0-9]};
my $rxNonDigit = qr{[A-MO-Z]};

foreach my $rhTest (@tests)
{
    (my $myAns = $rhTest->{str}) =~
       s{((?:$rxDigit)+$rxNonDigit)(?!\z)}{$1_}g;

    print
       qq{\n$rhTest->{str}\n},
       $myAns eq $rhTest->{ans}
       ? qq{OK\n}
       : qq{Not OK\n},
       qq{$myAns\n},
       qq{$rhTest->{ans}\n};
}
[download]

Here is the output.

423RY75Y69827EC67592C78657N965R
OK
423R_Y75Y_69827E_C67592C_78657N965R
423R_Y75Y_69827E_C67592C_78657N965R

423RY75Y69827EC67592C78657N965R345U299M
OK
423R_Y75Y_69827E_C67592C_78657N965R_345U_299M
423R_Y75Y_69827E_C67592C_78657N965R_345U_299M
[download]

I hope this is of use.

Cheers,

JohnGG