I think Perl is a little unique here. It's an acronym (therefore mistakely written as PERL) as well as being pronounceable, so written as Perl.
in reply to Re^2: PERL as shibboleth and the Perl community
in thread PERL as shibboleth and the Perl community
A language name like Python doesn't share this uniqueness, in the sense that hardly anyone would write PYTHON. It's not an acronym but the name of an animal. And becuase it's a proper noun, so it's unambiguously written Python.
PHP, on the other hand, is both an acronym and is unpronounceable. Consequently, it's rare to see it written as Php (I think).