As mentioned, the roll-your own with a mod 2 in there somewhere is a way to go. Could also do a more creative (read: maybe not best) join like:

my @datafiles = map { "file".$_ } 1..7;
printf "files available=\n%s\n",
  join("\n",                    # 4) glue w/your newlines
    grep { $_ }                 # 3) exclude blank elements created by
+ split()
      split(/([^,]+, [^,]+, )/, # 2) split on pairs
        join(", ", @datafiles)  # 1) glue big ,-delim string
      )
  )
;
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