this is called a hash slice.
someone has declared o as a hash, and wants to set up 3 elements in semi-odd way.
it's equivalent to 
%o= (a=>'a',
     b=>'b',
     c=>'c');

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except that you can't declare a hash under strict this way -- you still need to explicity code my %o, and then use the hash slice.

So what's it good for?
Primarily when you're populating an array from a hash, and you only want to use a subset of the hash's values. It can be onerous to type @ary=($hash{e}, $hash{f}, ... $hash{n}), if you only wanted those particular elements, so the almight Wall-Oz gave us the hash slice, which reduces the assignment to @ary=@hash{"e".."n"} (yes, there are other ways of doing this, I acknowledge)
Here's a small example for you showing the hash slice in action :

use strict;
my %o;
@o{qw(a b c d e f)} = (1,2,3,4,5);
my @foo;
@foo=@o{"c".."e"};
print @foo;

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the third line uses a hash slice to assign 5 values to %o. the fifth line uses a hash slice to easily take 3 elements from %0, and assign their values into @foo.
Also, even though I haven't demonstrated it, using 
@o("a","e","f")

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is just as legal.

See also array slices for a similar concept, but with arrays.

update : hey, neat! @foo=@bar{sort keys %bar};

In reply to How I learned to stop worrying and love the hash slice. (boo) by boo_radley
in thread beyond me by jroberts

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