The attribute specification :default('') didn't work as expected . Here is my program. 
#!/usr/bin/perl -w

package Foo;
{
use Class::Std;

my %a : ATTR( :default('') :get<a>);
};

my $f = Foo->new();

print ref($f), "\n";
if (defined($f->get_a())) { print "yes\n"; } else { print  "no\n"; }
if ($f->get_a() eq '') { print "yes\n"; } else { print "no\n";  }

package Bar;
{
use Class::Std;

my %a : ATTR( :default('''') :get<a>);
};

my $g = Bar->new();

print ref($g), "\n";
if (defined($g->get_a())) { print "yes\n"; } else { print  "no\n"; }
if ($g->get_a() eq '') { print "yes\n"; } else { print "no\n";  }

print "Hello\n";

if (undef eq '') { print "yes\n" } else { print "no\n"; }

[download]
When executed, this perl script gives the following output. 
./test14.pl
Foo
no
Use of uninitialized value in string eq at ./test14.pl line 14.
yes
Bar
yes
yes
Hello
Use of uninitialized value in string eq at ./test14.pl line 31.
yes

[download]
Based on the explanation of Class::Std in CPAN, I expected $f->get_a() to be the null string. However, according to the 2nd and 3rd line of output, it is actually undefined.

I have a conjecture. If you look at the source, you see that the given default value is first evaluated, after the string delimiters are removed. in the Foo case, eval '' returns normally with the undefined value. To initialize the hash bucket to the null string in Goo, I had to do :default(''''). Is this a bug?

In reply to Class::Std :default semantics by esharris

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