YAML vs YAML::XS - space needed after :

[szabgab]

A case where YAML 1.15 would accept something that YAML::XS 0.59 would not.
A minimal version of it:

use 5.010;
use strict;
use warnings;

use Data::Dumper qw(Dumper);
use YAML;
use YAML::XS;

my $data = <<'END';
x:
  'a': 'a'
  'b':'b'
END

my $x = YAML::Load($data);
say "ok YAML $YAML::VERSION";  # YAML 1.15 parses properly
#say Dumper $x; 
my $y = YAML::XS::Load($data);
#say "ok YAML::XS $YAML::XS::VERSION"; # 0.59

# Following error:

# YAML::XS::Load Error: The problem:
# 
#     could not find expected ':'
# 
# was found at document: 1, line: 4, column: 1
# while scanning a simple key at line: 3, column: 3