First of all, if you have two separate blocks of code you want to post, in this case a script and a data file, please use two separate code blocks.

Meshing it all into a single file makes it much harder for me to debug this thing. Maybe for YAML data is misformatted? The line numbers in the error message don't match up with the code you posted.

You don't use strict; use warnings;.

You essentially don't check for errors when reading the data file.

Other than that, at least on my computer it works, as long as a (more or less) correctly formatted input file is provided.

I too get that error if reading from test.yaml fails. Then $yaml silently becomes undef, the later use of $yaml->[0] autovivifies it into an array reference, and then it's not a blessed reference, and calling write on it fails.

I suggest to use the LoadFile and DumpFile functions instead, which seem to do proper error checking.

On using LoadFile instead of read, it gives:

"Failed to load YAML document from 'YAML::Tiny' at yaml_test.pl line 7
+"
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UPDATE The same code is working fine on unix machine, while on windows it's giving the same error. Not getting exactly what is the reason behind this. I checked Tiny.pm but there also nothing was found. Perl code:

use strict;
use warnings;
use YAML::Tiny;
use Data::Dumper;

# Create a YAML file
my $yaml = YAML::Tiny->new;

# Open the config
$yaml = YAML::Tiny->read( 'test.yaml' );

#changing yaml file
$yaml->[0]->{'order.create'}->{domain} = 'ckjckcjckj';

# Save the file
$yaml->write('test.yaml');
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Yaml File:

---
order.create:
  domain: sdfsdfsdfsdf
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The same code works in unix but gives error in windows "Can't call method "write" on unblessed reference at yaml_test.pl line 16.".

Based on your error message, I guess you called LoadFile as a method, not as a subroutine. But it's hard to know for sure without seeing any code.

Perl 6 - the future is here, just unevenly distributed

In my code I use use YAML qw{LoadFile}, and it works fine on both Windows and *nix. YAML file example:

---                                                      
version: 0.11
greek:
 - greek_plain
 - greek_italic
csr:                                                      
 FontStyle:                                               
  Italic: italic
 Position:
  Subscript: sub
  Superscript: sup
gs:
 - EPS
 - PDF
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Can you try running perl debugger on your code and using s command after LoadFile call shows up to trace code inside LoadFile?
Sorry if my advice was wrong.

$yaml = YAML::Tiny->read( 'D:\Perl\bin\test.yaml' )
  or die YAML::Tiny->errstr;
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Please show the exact code you're running. The symptoms you describe sound as if you're running

 my $string = Dump(...);
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without the appropriate use YAML::Tiny 'Dump'; at the top of your program. The documentation states that just three lines above your linked part of the documentation.

For anyone else that finds this post when searching for a solution to this issue... For me it was because I had spaces before the '---' in my config file so the yaml object wasn't being created. It would be useful to have a check and output YAML::Tiny->errstr in the example for n00bs like myself :)