https://www.socialtext.net/perl5/optree_guts

Options:
m   needs stack mark
f   needs constant folding
s   produces a scalar
i   produces an integer
t   needs a target
T   target can be in a pad
I   has a corresponding integer version
d   has side effects
u   uses $_ if no argument given


Values for the operand types are:
S   scalar
L   list
A   array
H   hash
C   sub (CV)
F   file
Fs  socket
F-  filetest
R   reference
"?" denotes an optional operand.

