m/.../ returns true or false in scalar context 
m/.../g give you a list of matches in list context 
m/(...)/ can populate an array with whatever gets captured... (??)
 
while( ) i.e. inside parenthesis is evaluated in scalar context? 
foreach( ) i.e. inside parenthesis is evaluated in list context?
 
<STDIN> returns the next line of input in list context
[download]

Well ... most of this can be tested interactively.

The inside context with wantarray the outside context behavior with print, print scalar and a void call.

For instance I doubted that scalar m// returns true or false (those identifiers don't exist in Perl), I thought it's the number of matches returned.

But I was wrong: :)

  DB<23> $_="aaa"
 
  DB<24> print scalar m/a/
1
[download]

UPDATE: Forgot to mention, you can also check the prototype for most builtins:

 perl -e 'print prototype "CORE::push","\n"'
\@@
[download]

And some builtins (like map) have such a "rich" syntax that they can't be expressed with prototypes.