> $in = '423RY75Y69827EC67592C78657N965R';;
> ( $out = $in ) =~ s[((\D)[0-9N]+\2)][_$1_]g;; 
> print $out;;
423R_Y75Y_69827E_C67592C_78657N965R

##</code><code>##

> $in = '423RY75Y69827EC67592C78657N965R345U299M';;
> ( $out = $in ) =~ s[ 
    ( ( \D ) [0-9N]+ \2 ) 
  | 
    ( [0-9N]+? [A-MO-Z] (?=\d) ) 
]{ defined $1 ? "_$1_" : "$3_" }egx; 

print $out;;
423R_Y75Y_69827E_C67592C_78657N965R_345U_299M