Question

<>;
print $_

But this works:

while(<>){
  print $_;
}

Why doesn’t the first version work?

Answer 1

Because while (<>) is shorthand for while(defined($_ = <ARGV>)), but <>; is just <ARGV>;. You can see this with the B::Deparse module.

Given the file example.pl

#!/usr/bin/perl

use strict;
use warnings;

<>;

while (<>) {
}

the command

perl -MO=Deparse example.pl

will print

use warnings;
use strict 'refs';
<ARGV>;
while (defined($_ = <ARGV>)) {
    ();
}
example.pl syntax OK

This is an example of Perl’s famous DWIMery. I believe it was done to stop people from doing the wasteful

for (<>) {
}

There is no value to special casing <> in void context, and, in fact, it would probably be harmful since when you say <>; you are most often trying to throw away a line you don’t want from the filehandle. Having $_ catch it would not be efficient or desirable.