Really weird perl syntax error adding TO_JSON method to package - json

Question

In adding a TO_JSON method (to convert a blessed reference via JSON.pm) into CGI::Cookie if I do this:
package CGI::Cookie;
sub TO_JSON {
return {
map { name => $_->name,
value => $_->value,
domain => $_->domain,
path => $_->path,
expires => $_->expires }
shift
}
}
syntax error at XXX.pm line 76, near "shift " syntax error at XXX.pm
line 77, near "}" Compilation failed in require at (eval 50) line 3.
But if I do this:
package CGI::Cookie;
sub TO_JSON {
return {
map { ''.'name' => $_->name,
value => $_->value,
domain => $_->domain,
path => $_->path,
expires => $_->expires }
shift
}
}
it works
Can't for the life of me figure out why. Also just quoting "name" doesn't help. I have to concatenate an empty string for it to work.
I'm mystified.

Answer 1

The Perl grammar is a bit ambiguous when it comes to blocks and anonymous hashrefs. When Perl cannot guess correctly, you can force the correct interpretation:
Hashref by +{ ... }
Codeblock by {; ... }
Forcing the block after map to be a codeblock resolves the issue. Previously it thought the block was an anonymous hash, and missed a comma before the shift: map can be of the form map EXPR, LIST, and a hashref is a valid expression.
The sub uses misuses map to assign one element to $_. It would better be written:
sub TO_JSON {
my $o = shift; # my $_ should work as well, but that is beside the point
return +{
name => $o->name,
value => $o->value,
domain => $o->domain,
path => $o->path,
expires => $o->expires,
};
}
But it could be abbreviated to
sub TO_JSON {
my $o = shift;
return +{
map { $_ => $o->$_() } qw/name value domain path expires/
};
}