If you are on Microsoft Windows using NTFS then you can use an Additional Data Stream (ADS), which is used for exactly your purpose with, for example, Notepad. ADS streams are marked with a :suffix. For example, from cmd.exe:

echo secret > fred.txt:hidden
more < fred.txt:hidden
secret
[download]

The hidden ADS is otherwise not normally visible. In Perl you can open, read, and write, ADS files in the same way as any other.