So this is as far as I got and I'm stymied:

#!/usr/bin/env perl
use strict;
use warnings;

my $text = "blah blah [click me](click me) more stuff\nblah [link here
+](link here) blah blah";
$text =~ s/(\[[^]]+]\()/$1\//g; # make links absolute
$text =~ /(\[[^]]+]\(\/)([^\)]+)/;
my $part1 = $1;
my $part2 = $2;
my $orig_part2 = $part2;
$part2 =~ s/ /-/g;
$text =~ s/\Q$part1$orig_part2\E/$part1$part2/g;
[download]

The obvious problem with this code is that only the first link gets the space replaced:

blah blah [click me](/click-me) more stuff
blah [link here](/link here) blah blah
[download]

I'm drawing a blank on how I might loop through all matches to links and modify those matches. Or maybe there is an entirely better way of pulling this off. Thanks!

UPDATE:

OK, I noodled around with this some more. On a lark, I tried a "global" match (which I didn't know existed outside substitution), and came up with this:

my $text = "blah blah [click me](click me) more stuff\nblah [link here
+](link here) blah blah";
$text =~ s/(\[[^]]+]\()/$1\//g; # make links absolute
my @matches = $text =~ /(\[[^]]+]\(\/)([^\)]+)/g;
while (@matches) {
  my $part1 = shift @matches;
  my $part2 = shift @matches;
  my $orig_part2 = $part2;
  $part2 =~ s/ /-/g;
  $text =~ s/\Q$part1$orig_part2\E/$part1$part2/g;
}
[download]

Though it seems to work, I have a hunch this isn't ideal.