my $L = '[A-MO-Za-mo-z]';
my $D = '[0-9N]';

$_ = '423RY75Y69827EC67592C78657N965R345U299M';
s/($L?)($D+)\1/_${1}${2}${1}_/g;         
print $_, "\n";
__END__
_423_R_Y75Y__69827_E_C67592C__78657N965_R_345_U_299_M