Interesting. Normally, when you type echo in a shell, its echo builtin is being used (I think this applies to OS X, too).

Initially, I suspected a difference between the echo shell builtin and /bin/echo to be the cause here, but then, on second thought, rejected the hypothesis, as you said you were getting the correct digest when issuing the openssl command on the command line with the same plain "echo".

In other words, as I see things, with a plain "echo" you should be using the builtin in both cases, i.e. from within the backticks (because of the '|' symbol, which should cause Perl to run the command via the shell (sh -c ...)), as well as on the interactive command line (which is a shell, anyway, typically).  So I'm still wondering what's different here on OS X.  Any ideas, anyone?