use Encode qw(decode);

my $str = 'username, A Ne=?UTF-16?B?dwAgAEMAcgBlAGQAaQB0ACAAQwBhAHIAZAAgAEMAbwB1AGwAZAAgAEIAZQAgAEgAZQBhAGQAZQBkACAAWQBvAHUAcgAgAFcAYQB5AA==?=';

my $chr = decode('MIME-Header', $str);

print "$chr\n";