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s/([^[:^alpha:]nN])([\dnN]+\1)/_$1$2_/g;
s/__/_/g;

my $string = '423RY75Y69827EC67592C78657N965R345U299M';
my $correct= '423R_Y75Y_69827E_C67592C_78657N965R_345U_299M';
...
    s/_(_|$)/$1/g;
}
print "Okay\n"   if  $string eq $correct;