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

...

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

s[
    (?<!^)  # not preceded by the start of string
...
    \t      # replace tabs
    (?!"|$) # that are also not followed by a double quote or the end 
+of string
][***]gx;