Good idea! I just wrote the following and am executing it as we speak (it's at 1_400_000 right now):

#!/usr/bin/perl 
 
my $l=600_000; 
 
while (1) { 
  print "\rTesting $l..."; 
  open OUT,">biglines.txt" || die; 
  print OUT "x" x $l; 
  close OUT; 
 
  open IN,"<biglines.txt" || die; 
  my $r=<IN>; 
  close IN; 
 
  if (length($r)!=$l) { 
    print "Oops! A linelength of $l fails!!\n"; 
    exit; 
  } 
  $l+=5000; 
}
[download]

It doesn't have good resolution, but it would take it years to try all the possible lengths.

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(12:01pm EST) Addendum: well, it's up to 25_000_000 characters on a line and still no problems!

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(2:28pm EST) Well, I finally stopped it from executing at approximately 65MB per line :) At this point I'd say that there is no theorectical line-length-limit, at least under Linux. I would be interested, though, to see if Perl under NT lacks the same limitation.