That explains a lot! My problem is that the squiggly (aka tilde) is NOT interpreted within SINGLE quotes.

The tilde ~ is in general not special to Perl. To get the home directory, there's e.g. File::HomeDir, or you could use glob - but be aware that it'll treat any glob metacharacters as special, and there are several caveats when it comes to glob, which is why you might be interested in Path::ExpandTilde instead.

What about the shell? Is it the login shell (Bash) or the shell the Perl source is invoked from (tcsh) or just Linux?

That's not always clear, which is one of the reasons I usually recommend to avoid the shell altogether.