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

##</code><code>##

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

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

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

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