Slick! I wouldn't have thought of using reduce for this. I'll ++ you tomorrow when I get more votes ;)
There is a small bug in the implementation, though: single number ranges fail. Add 618 or 619 to the test data, you get "618 isn't there!".
Here's a patch to fix it:
# $last ||= 1; # needs to be
$last ||= $first;
And here's another patch that disposes of pre-allocating the 1000 character string for
$zones. I'm sure it's slower, but it'll work for an arbitrary range (ie, > 1000):
my $zones = reduce {
my( $first, $last ) = split '-', $b;
$last ||= $first;
#allocate $a here
$a .= chr(0) x ( $last );
substr( $a, $first, $last - $first + 1 ) = 'x' x ( $last - $first
++ 1 );
$a
} '', split ',',
'10-374,376-379,382-385,388-499,530-534,541-543,618,619,700-704,707-70
+9';
Update: added second patch
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