Another way to do it... (but only works on newer perls with lookbehind.) I didn't realize it was a FAQ when I needed to do it. The theme is the same though. Repeatedly make substitutions until there are none left to make.

1 while ( s/(?<!\b)(\d{3})(?:,|$)/,$1/ );

# Explained
1 while (             # Repeat until no more matches are made.
    s/(?<!\b)(\d{3})  # Match any three digits not preceded by a word 
+break. 
      (?:,|$)         # That come immediately before a comma or the en
+d.
     /,$1/x           # And insert a comma before them. 
);
[download]

-sauoq
"My two cents aren't worth a dime.";