Perl is seeing the pipe as alternation to nothing, i.e. 'either this or anything else'. Here are the details from a debugging perl (5.8.0-RC1):

$ perl -Dr pipe.pl
Omitting $` $& $' support.

EXECUTING...

Compiling REx `lo/gos'
size 4 Got 36 bytes for offset annotations.
first at 1
rarest char / at 2
   1: EXACT <lo/gos>(4)
   4: END(0)
anchored `lo/gos' at 0 (checking anchored isall) minlen 6 
Offsets: [4]
    1[6] 0[0] 0[0] 7[0] 
Guessing start of match, REx `lo/gos' against `lo/gou'...
Did not find anchored substr `lo/gos'...
Match rejected by optimizer
Freeing REx: `lo/gos'
Compiling REx `lo/gou'
size 4 Got 36 bytes for offset annotations.
first at 1
rarest char / at 2
   1: EXACT <lo/gou>(4)
   4: END(0)
anchored `lo/gou' at 0 (checking anchored isall) minlen 6 
Offsets: [4]
    1[6] 0[0] 0[0] 7[0] 
Guessing start of match, REx `lo/gou' against `lo/gou'...
Found anchored substr `lo/gou' at offset 0...
Guessed: match at offset 0
lo/gou
Freeing REx: `lo/gou'
Compiling REx `lo/gw|'
size 7 Got 60 bytes for offset annotations.

   1: BRANCH(5)
   2:   EXACT <lo/gw>(7)
   5: BRANCH(7)
   6:   NOTHING(7)
   7: END(0)
minlen 0 
Offsets: [7]
    0[0] 1[5] 0[0] 0[0] 6[1] 6[0] 7[0] 
Matching REx `lo/gw|' against `lo/gou'
  Setting an EVAL scope, savestack=16
   0 <> <lo/gou>          |  1:  BRANCH
  Setting an EVAL scope, savestack=22
   0 <> <lo/gou>          |  2:    EXACT <lo/gw>
                              failed...
   0 <> <lo/gou>          |  6:    NOTHING
   0 <> <lo/gou>          |  7:    END
Match successful!
lo/gw|
Freeing REx: `lo/gw|'
Compiling REx `lo/gon'
size 4 Got 36 bytes for offset annotations.
first at 1
rarest char / at 2
   1: EXACT <lo/gon>(4)
   4: END(0)
anchored `lo/gon' at 0 (checking anchored isall) minlen 6 
Offsets: [4]
    1[6] 0[0] 0[0] 7[0] 
Guessing start of match, REx `lo/gon' against `lo/gou'...
Did not find anchored substr `lo/gon'...
Match rejected by optimizer
Freeing REx: `lo/gon'
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After Compline,
Zaxo

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