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
}