ASCII space conveniently has only the lowercase-bit set. If tye hasn't told you yet, this is some code to remember and set that bit:

for (<DATA>){
  chomp;
  $case = $_ & ' ' x length;
  s/old/NEW/i;
  print $_ | $case. "\n";
} 

__DATA__
old_word
OLD_WORD
oLd_wOrd
olD_worD
[download]

Cheers,

Jeroen
"We are not alone"(FZ)