In your prior post, lower processing time was the goal, not an artificial avoidance of loops not generated within the regex engine. See my reply Re: How do I match lines of 40 characters long in a block of text? for an efficient loop.

Accepting that this is homework or a bar bet; if you must use a regex to decide that a block of text has fewer than five short lines, try to match five or more - failure is success:

if ($foo !~ m/\A(?:             #anchor start and group
                 (?:.{41,}\n)*  #maybe some long lines 
                 (?:.{0,40}\n)  #one short line
               ){5,}            #five or more
               (?:.{41,}\n)*    #maybe some more long lines 
               \z               #anchor end
             /x ) {
    #do something
}
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Update: Fixed link. Your first regex may work if you anchor the ends. It is matching long lines by backtracking to the -40 position from the newline.

After Compline,
Zaxo