why does perl while (<>) fail to count or print the first line - perl

Question

I want to count the lines in a file and print a string which depends on the line number. But my while loop misses the first line. I believe the while (<>) construct is necessary to increment the $n variable; anyway, is not this construct pretty standard in perl?
How do I get the while loop to print the first line? Or should I not be using while?
> printf '%s\n%s\n' dog cat
dog
cat
> printf '%s\n%s\n' dog cat | perl -n -e 'use strict; use warnings; print; '
dog
cat
> printf '%s\n%s\n' dog cat | perl -n -e 'use strict; use warnings; while (<>) { print; } '
cat
>
> printf '%s\n%s\n' dog cat | perl -n -e 'use strict; use warnings; my $n=0; while (<>) { $n++; print "$n:"; print; } '
1:cat

Answer 1

The man perlrun shows:
-n causes Perl to assume the following loop around your program, which makes it iterate over filename
arguments somewhat like sed -n or awk:
LINE:
while (<>) {
... # your program goes here
}
Note that the lines are not printed by default. See "-p" to have lines printed. If a file named by an
argument cannot be opened for some reason, Perl warns you about it and moves on to the next file.
Also note that "<>" passes command line arguments to "open" in perlfunc, which doesn't necessarily
interpret them as file names. See perlop for possible security implications.
...
...
"BEGIN" and "END" blocks may be used to capture control before or after the implicit program loop, just as in awk.
So, in fact you running this script
LINE:
while (<>) {
# your progrem start
use strict;
use warnings;
my $n=0;
while (<>) {
$n++;
print "$n:";
print;
}
# end
}
Solution, just remove the -n.
printf '%s\n%s\n' dog cat | perl -e 'use strict; use warnings; my $n=0; while (<>) { $n++; print "$n:"; print; }'
Will print:
1:dog
2:cat
or
printf '%s\n%s\n' dog cat | perl -ne 'print ++$n, ":$_"'
with the same result
or
printf '%s\n%s\n' dog cat | perl -pe '++$n;s/^/$n:/'
but the ikegami's solution
printf "one\ntwo\n" | perl -ne 'print "$.:$_"'
is the BEST

Answer 2

There's a way to figure out what your one-liner is actually doing. The B::Deparse module has a way to show you how perl interpreted your source code. It's actually from the O (capital letter O, not zero) namespace that you can load with -M (ikegami explains this on Perlmonks):
$ perl -MO=Deparse -ne 'while(<>){print}' foo bar
LINE: while (defined($_ = readline ARGV)) {
while (defined($_ = readline ARGV)) {
print $_;
}
-e syntax OK
Heh, googling for the module link shows I wrote about this for The Effective Perler. Same example. I guess I'm not that original.
If you can't change the command line, perhaps because it's in the middle of a big script or something, you can set options in PERL5OPT. Then those options last for just the session. I hate changing the original scripts because it seems that no matter how careful I am, I mess up something (how many times has my brain told me "hey dummy, you know what a git branch is, so you should have used that first"):
$ export PERL5OPT='-MO=Deparse'