What's the difference between ""perl"" and ""Perl""?

One bit.  Oh, you weren't talking ASCII? :-) Larry now uses "Perl" to signify
the language proper and "perl" the implementation of it, i.e. the current
interpreter.  Hence Tom's quip that "Nothing but perl can parse Perl."  You
may or may not choose to follow this usage.

[...]

But never write "PERL", because perl isn't really an acronym, aprocryphal
folklore and post-facto expansions notwithstanding.