Ignoring your requirements and just looking at your example, I derive a different set of rules:

1. Treat an "N" as if it was a digit
2. If there's a group of letters (minus "N") followed by a digit (or "N") somewhere down the road, then put a underscore after the first letter.

In code:

#! perl -wl
my $string = '423RY75Y69827EC67592C78657N965R345U299M';
my $correct = '423R_Y75Y_69827E_C67592C_78657N965R_345U_299M';
$string =~ s/([A-MO-Z])([A-MO-Z]*[\dN]+)/$1_$2/g;
print $string eq $correct' ? 'OK' : 'NOT OK';
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This prints:

OK

Are you sure you've got your requirements right?