I want to use a flip-flop to truncate a list after a certain value. This should be easy, right? After all, getting a sub-sequence works: 
  # Get everything between B and D:
  perl -lwe 'print join " ", grep scalar(/B/../D/), qw(A B C D E F)'
  # prints:
  # B C D

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This doesn't work, because of the (documented in perlop) magic comparison with $. when the conditions are constants: 
  # Get everything between up to D:
  perl -lwe 'print join " ", grep scalar(1../D/), qw(A B C D E F)'
  # prints:
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.
  # Use of uninitialized value in range (or flip) at -e line 1.

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Ok, fair enough. But this doesn't work correctly, and there aren't any warnings: 
  # Get everything up to and D:
  perl -lwe 'print join " ", grep scalar(/./../D/), qw(A B C D E F)'
  # prints:
  # A B C D E F

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In fact, as flip-flops return the sequence numbers of their matches, we can double check what these are: 
  # Get everything up to D:
  perl -lwe 'print join " ", map scalar(/./../D/), qw(A B C D E F)'
  # prints:
  # 1 2 3 4E0 1 2

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Eh? Why does it start back at 1 after 4E0?

Can anyone shed any light on this? (I'm using Perl 5.8.8 on Ubuntu.)

In reply to Flip-flop won't DWIM by wu-lee

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