$ perl
no more Perl for me, unless less than 40 lines
syntax error at - line 1, near "Perl for "
Execution of - aborted due to compilation errors.
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$ perl 'no more Perl for me, unless less than 40 lines'
Can't open perl script "no more Perl for me, unless less than 40 lines
+": No such file or directory

$ perl -w
`no more Perl for me, unless less than 40 lines`
^D
Can't exec "no": Permission denied at - line 2.

$ perl -e "`no more Perl for me, unless less than 40 lines`"
-bash: no: command not found

$ which no
which: no no in (/usr/local/bin:/usr/bin:/bin:~/bin:/opt/bin)
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;-)

We're building the house of the future together.

  do { unlink('perl') for ($me); } unless (__LINE__ < 40);
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'no more Perl for me, unless less than 40 lines'

package ACME::WeightWatchers;
use Carp;
require Exporter;
our @ISA    = qw(Exporter);
our @EXPORT = 'watchsize';

sub watchsize {
  local $/ = "\n";
  my $caller = caller;
  open _, "$caller::$0";
  while (<_>) {
    $. == 40 
      and confess "no more Perl for me, unless less than 40 lines\n";
  }
}
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#!/usr/bin/perl
use strict;
use warnings;
use ACME::WeightWatchers;

watchsize();

# your code here ...
print "I made it under 40!\n";
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no more Perl for me, unless less than 40 lines

Which is plenty of space. I mean, 40 chars would have been a restriction, but 40 lines?

I love oneliners!