$s = "\tfred\tx\t\"abc\tdef\"\ty\tz\t";;

print "'$s'";;
'       fred    x       "abc    def"    y       z       '

$s =~ s[(?<!^)(?<!")\t(?!"|$)][***]g;;

print "'$s'";;
'       fred***x        "abc***def"     y***z   '

##</code><code>##

s[
    (?<!^)  # not preceded by the start of string
    (?<!")  # nor by a double quote
    \t      # replace tabs
    (?!"|$) # that are also not followed by a double quote or the end of string
][***]gx;