no, unfortunately there's no prototype mechanism for the latter.

You can neither easily allow calling a function (which is a special case of the expression part).

There was a discussion about this a year ago, I will update a link.

UPDATE:

see coderefs and (&) prototypes and The & prototype and code references in scalars..

here's a reimplementation of the Perl grep operator:

sub mygrep (&@) {
    my $code = shift;
    my @result;
    foreach $_ (@_) {
        push(@result, $_) if &$code;
    }
    @result;
}
[download]

Update: Your example would be called as: print join ",", mygrep {$_ % 2 == 0} 0..10; Note the deviation from the proposed use case. I'd initially missed the spec for "automatically put [ting] it in an implicit block"

That won't work, for the proposed case:

Type of arg 1 to main::mygrep must be block or sub {} (not numeric eq 
+(==)) at /tmp/mygrep line 7, near "10)"
Execution of /tmp/mygrep aborted due to compilation errors.
[download]

Perhaps something like this?

sub mygrep { my $s = "grep $_[0]"; eval $s; }

print for mygrep('$_ % 2 == 0, 0..10');    # :-)
[download]


--
"Language shapes the way we think, and determines what we can think about."
-- B. L. Whorf

I added a use case, as this sort of structure is a bit sensitive - perl apparently interprets the OP proposed use case as an anonymous hash because parentheses are present.

When I was going through a clever phase, I played a bit with prototypes, and decided they are generally more trouble than they are worth. IMHO, of course.

Interesting, looks like how I imagine Devel::Declare might work ...

http://perldoc.perl.org/5.12.1/perlapi.html#PL_keyword_plugin

but:

 NOTE: this function is experimental and may change or be removed without notice.

Cheers Rolf