uninitialized value $

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I know I am missing some very basic concepts (but that is why I am here). Trying to create a simple program that echoes entered text to a file (and stops when a blank line is entered). So I wrote:

#!/usr/bin/perl
use strict;
use warnings;

my $file = "/path/to/some/file";

open ( FILE, '>', $file )
    or die "Can't open $file for writing.\n";

while ()
    {
    <>;
    last unless /\S/;
    print FILE;
    }

close FILE
    or die "Can't close $file.\n";
[download]

But that just gets me:

Use of uninitialized value $_ in pattern match (m//) at...
[download]

Where and how do I initialize that?

Comment on uninitialized value $_ Select or Download Code

Replies are listed 'Best First'.
Re: uninitialized value $_ by Corion (Patriarch) on Mar 28, 2016 at 16:21 UTC
See readline. If the filehandle (or STDIN in your case) gets closed, readline returns `undef`. Your loop is easier written in Perl as: `while( <> ) { last unless /\S/; print FILE; };` [download]	[reply] [d/l] [select]
Re^2: uninitialized value $_ by Anonymous Monk on Mar 28, 2016 at 17:04 UTC
Are you saying that STDIN is getting opened and closed every time a line is read? I guess that would explain why there is nothing there. :) How about a small variation where I need to print something (a prompt or line number) on the terminal before reading the line (so I couldn't have <> in the loop control)? The (broken) code would be something like: `while () { print $foo; <>; last unless /\S/; print FILE; }` [download]	[reply] [d/l]
Re^3: uninitialized value $_ by Paladin (Vicar) on Mar 28, 2016 at 17:16 UTC
`<>;` just reads a line and throws it away. Perhaps you meant `$_ = <>;`?	[reply] [d/l] [select]
Re^4: uninitialized value $_ by Anonymous Monk on Mar 28, 2016 at 18:42 UTC
Re^5: uninitialized value $_ by LanX (Saint) on Mar 29, 2016 at 00:08 UTC
Some notes below your chosen depth have not been shown here
Re^5: uninitialized value $_ by Marshall (Canon) on Mar 28, 2016 at 22:26 UTC
Re: uninitialized value $_ by shadowsong (Pilgrim) on Mar 31, 2016 at 15:50 UTC
In addition to what Corion has imparted; once you understand what was causing the error - here is a one-liner you may use that echoes input to `filename.txt` and stops when a blank line is entered: `perl -pe "last if /^\s*$/" > filename.txt` [download]	[reply] [d/l] [select]