There are two regex modifiers that help. /s so that . matches \n and /m so that ^ and $ match at the start and end of logical lines. /x allows free formating and embedded comments.

Using these modifiers you regex can be written as

my @array = $line =~ m{
    ^(LENGTH:     # a line beginning with LENGTH:
    .*?)          # as little as possible until
    ^(SUBJECT:    # a line beginning with SUBJECT:
    .*?)          # as little as possible until
    ^(COMMENT:    # a line beggining with COMMENT
    .*)           # and the rest 
}msx
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which will create an array with

$VAR1 = [
          'LENGTH: ...................................................
+...
..................................................................
..................................................................

...................................................................
..................................................................

',
          'SUBJECT: ..................................................
+.....

',
          'COMMENT: ..................................................
+...
....................................................................
'
        ];
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