Keep in mind, if you're using perl -ne, ARGV never gets closed, just reopened on every file on the command line, so if you're doing the one-liner across multiple files, you may not get the expected results. (See perlvar)

Here's what I mean (the first line uses bash to set up the test case...)

for i in 1 2 3 4 5; do perl -e'print "$_\n" foreach 1..$ARGV[0]' $i > 
+$i;done
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Once you run that, you've got 5 files in the current directory, 1..5, where 1 contains 1 line, and 5 contains 5...

perl -ne'printf "File: $ARGV \$.: %2d real line: $_",$. if $. & 1' 1 2
+ 3 4 5
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This results in:

File: 1 $.:  1 : real line: 1
File: 2 $.:  3 : real line: 2
File: 3 $.:  5 : real line: 2
File: 4 $.:  7 : real line: 1
File: 4 $.:  9 : real line: 3
File: 5 $.: 11 : real line: 1
File: 5 $.: 13 : real line: 3
File: 5 $.: 15 : real line: 5
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So in file 2 and 3, you get the even lines, because file 1 had an odd # of lines. If we had a file 6, its even lines would get printed instead of the odd ones, since the (1+2+3+4+5) is an odd #.

Fun, eh? :)