All,
I am pretty sure this question has been asked before but my Super Search fu is weak.

The Dictionary:

Use the official '2of12inf.txt' file from here stripped of all non-alpha characters and forced to lowercase.

while (<$dict_fh>) {
    $_ = lc($_);
    tr/a-z//cd;
    # ...
}
[download]

The Input:

The code should accept a file name as a command line argument. The file will contain a list of strings, one on each line. The string needs to be forced to lowercase with all non-alpha characters removed like the dictionary above.

penisland
howareyou
[download]

The Output:

For each line of input, the code should output all possible substrings (separated by whitespace) with the following constraints:

• Substrings should appear in the order they appear in the string
In other words, the substrings stripped of whitespace should be the input string
• Every substring should be either exactly a word from the dictionary or a substring containing no word from the dictionary as a further substring
"ehim" is an invalid substring because it contains "eh", "hi" and "him" as substrings.
• All substrings that do not contain a word from the dictionary should be as long as possible
"wz" followed by "gt" is not valid because "wzgt" does not contain any word from the dictionary.
• The list should be ordered with the fewest number of non-word characters then by fewest number of total substrings

Example:

penisland

pen island
penis land
pen is land
penis l land
pen is l and
[download]

For a somewhat similar challenge, see One for the weekend: challenge. Note: The example above was done by hand and could possibly not include all possibilities.

Update: Clarified constraint two thanks to /msg from ikegami
Update: Clarified example thanks to /msg from blokhead and ikegami

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