It all began when I was playing with indirect filehandles. I have a small code below where I want to simply open an input file and print its content into an output file.

#!/usr/bin/perl

use warnings;
use strict;

open( my $output, '>', '/tmp/output.txt' )
  or die "Error: Cannot open the file - $!\n";

open( my $input, '<', '/tmp/input.txt' )
  or die "Error: Cannot open the  file - $!\n";

print {$output} while (<$input>);   ## This is were the problem is.

close( $input );
close( $output );
[download]

When I run this code, I get this error.

syntax error at test.pl line 12, near "} while"
Execution of test.pl aborted due to compilation errors.
[download]

This is referring to the line print {$output}...
If I change this line to remove the brackets between the variable $output,

print $output while (<$input>);
[download]

And running it, I get this on the standard output, and no output file is created.

GLOB(0x84f3bdc)GLOB(0x84f3bdc)....
[download]

Finally, when I change the line to this...

print {$output} $_ while (<$input>);
or
print $output $_ while (<$input>);
[download]

Then everything is okay. My question is why do I have to explicitly write $_ in the above cases? I would assume that the content of $_ is changing, but I'm not sure where and I'm not sure if this assumption is even correct.

And compare this to the code below where I do not use indirect filehandles to write to an output file. Note that in the code below, I do not need to explicitly write $_ when printing the output. I was expecting that I can do the same with indirect filehandles.

#!/usr/bin/perl

use warnings;
use strict;

open( OUTPUT, '>', '/tmp/output.txt' )
  or die "Error: Cannot open the file - $!\n";

open( my $input, '<', '/tmp/input.txt' )
  or die "Error: Cannot open the  file - $!\n";

print OUTPUT while (<$input>);

close( $input );
close( OUTPUT );
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.