s/\bold\b/new/gi; 
#  1: replaces all instances of the string, e.g. "old", 
#  "OLD", "Old", "OlD" on either side with "new".

s/\bold\b/new/g; 
#  2: replaces "old" with "new" -- case sensitive here.

s/\bOLD\b/NEW/g; 
#  3: replaces "OLD" with "NEW"

s/\bOLD\b/NEW/gi; 
#  4: replaces same variants of "OLD" as in 1 with "NEW"