grep { s/user::/USER:$o:/} @A; # make native Linux dir/file permis
+sions uppercase
    grep { s/group::/GROUP:$g:/} @A; # idem
    grep { s/other::/OTHER::/} @A; # idem
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That is usually written as:

    s/user::/USER:$o:/ for @A; # make native Linux dir/file permission
+s uppercase
    s/group::/GROUP:$g:/ for @A; # idem
    s/other::/OTHER::/ for @A; # idem
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    $P{$k} =~ s/(?:user|group):[\s\S]*\K(other:.*)//mi;
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[\s\S]* could also be written as (?s:.*) and the /m modifier is only useful if the anchors ^ or $ are used.

    $P{$k} =~ s/[\n\r\s]+/ /g; # remove newlines
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The \s character class includes the characters \n and \r so that could be simplified to $P{$k} =~ s/\s+/ /g;.