I'm not able to come up with the origin of the word sigil, but I first saw it in Damian Conway's Exegesis 2,
and I know the term is used by gnat and japhy.
The term 'sigil' to mean the funny character at the front of a Perl variable
was coined on 3 March 1999 by Philip Gwyn. I immediately posted
about it to p5p:
The source code actually uses the term `funny characters' to refer to
the $@%*& characters that select the variable type. This terminology
leaves a lot to be desired, but until now, I'd never heard a better
suggestion. They don't really have a good name. This has sometimes
been a problem for me when I've written documentation. The temptation
is to pretend that the funny character is part of the variable name,
which it isn't.
Philip Gwyn just used the word `sigil'. This struck me as a really
wonderful coinage that shouldn't be lost. The spelling of `sigil'
suggests `sign', which is good, and the word doesn't presently have
any other meaning in the Perl context, also good.
Glancing over man pages like `perldata' I can see a lot of places
where the explanations would become simpler or more accurate if the
word `sigil' were used.
Finally, according to the dictionary here, it means:
A sign or image considered magical.
Which is a really perfect description of it.
The term started catching on right away, and has gradually gained popularity.