$n =~ s/(^| )$a( |$)/$1$b$2/i;

##</code><code>##

  $n =~ s/(^|(?<= ))$a((?= )|$))/$b/i;

##</code><code>##

  $n =~ s/(?<![^ ])$a(?![%a])/$b/i;

##</code><code>##

  "FOO foo" =~ s/(^| )foo( |$)/${1}bar$2/ig