Why use the -p|-n in slurp mode in perl one liner?

Question

In perl one liner slurp mode is 0777 I hope which is equal to below script

open my $fh, "<","file";
local $/;
my $s = <$fh>; #now whole file stored into the $s

Here we are not using any loop for storing all the elements together(single data).

But in perl one liner, Why do we use the -p|-n switch to enable the slurp mode (-0777)? What is the performance gain here.?

-p | -n using for looping purpose. So the actual performance of one liner is like the below script or anything else?

open my $fh, "<","file";
my $s;
while (<$fh>)
{ 
   $s.=$_;
}
print $s;

Answer 1

Without -n or -p there is no implicit while (<>) loop so the default $_ does not get set, and the STDIN isn't read either. If we want to have <> and $_ by default in the slurp mode we need one of these switches along with -0777, which on its own merely (un)sets $/. This

echo "hello" | perl -0777 -e 'print'

prints nothing, and with -w it warns of Use of uninitialized value $_. Now this

echo "hello" | perl -0777 -e '$v = <>; print $v'

does print hello. The STDIN can be read into a variable, so 'slurp' is on.

In terms of what this is equivalent to, mere -0777 only does $/ = undef. If we add a read

# use warnings;
local $/;
<>; 
print;

The <> does read everything in one go but there is no default input and pattern-searching space (variable $_) so what is read is not assigned to anything. With the warnings on we get to hear about it. (Thanks to Jonathan Leffler for mentioning STDIN in a comment.)

The code equivalent to using -n is

while (defined($_ = <ARGV>)) { }

so we get standard input and $_ set up. Run perl -MO=Deparse -n -e 1 to see these.

In perlvar the conditions are listed for when "... Perl will assume $_ ...". The last bullet

The default place to put the next value or input record when a <FH>, readline, readdir or each operation's result is tested by itself as the sole criterion of a while test. Outside a while test, this will not happen.

With -n or -p switches we get this, and all of standard input.

Note, your example is not exactly equivalent since it does assign.

---

Comment on specific statements in the question.

The slurp does not get "enabled" by these switches -- it is set up by the -0777 flag. We use them because we get automatic standard input and $_ with them.