item    -> var | special | label
var     -> ( label ':' )? classname? '<ro>'? (scalar |
           array | hash | glob | code | io | lvalue )
scalar  -> number | string | ref | 'undef'
ref     -> '<weak>'? ( '\' item | '[' list? ']' | '{' hlist? '}' )
array   -> '@(' list? ')'
hash    -> '%(' hlist? ')'
list    -> item (',' item)*
hlist   -> key '=>' item (',' key '=>' item)*
glob    -> '<anon>'? '*' package? name
code    -> '<format>'? '&(' filename ':' linenum ')'
io      -> '<io>'
lvalue  -> substr | pos | vec
substr  -> 'substr(' item ',' number ',' number ')'
pos     -> 'pos(' item ')'
vec     -> 'vec(' item ',' number ',' number ')'
special -> '<undef>' | '<yes>' | '<no>'