and this works

No, this not works:

my $data = {key => "THIS: "};
my $val = "HI THIS WORKS";
$data{key} .= $val;
print "$data{key}";
__END__
HI THIS WORKS
[download]

Why? cuz $data (SCALAR, may hold an anonymous hash) and %data (HASH) are different variables.

Read the perldata manual page. Seriously:

Read perldata.

update:

Reading perldata is not a waste of time. It saves you a lot of puzzling and doubts for time to come. Not reading it in turn wastes your and our time.