I was half expecting it might have been something obvious that I was missing.
Something in your reponse made me suddenly think "hang on ... I'm doing this over an ssh connection". So I decided to see whether the same problem existed when I sat at the computer's terminal (using identical path) and accessed it directly ... and it didn't !! Everyting works as expected when I'm sitting at the terminal.

But what makes it even weirder is that I have other perls under ~, and those perls *are* accessible as "perl" over the ssh connection.
For example:

sisyphus-sis@debian-sis:~$ echo $PATH
/home/sisyphus-sis/perl520-m64-multi/bin:/usr/local/bin:/usr/bin:/bin:
+/usr/local/games:/usr/games
sisyphus-sis@debian-sis:~$ which perl
/home/sisyphus-sis/perl520-m64-multi/bin/perl
sisyphus-sis@debian-sis:~$ perl -le 'print $^X'
/home/sisyphus-sis/perl520-m64-multi/bin/perl
sisyphus-sis@debian-sis:~$
[download]

No problems with that (or any of the 7 other installations of) "perl" over the ssh connection. It's just the bleadperl that won't play ball.
Thanks for the reply tobyink

Cheers,
Rob