Seems like you can save a couple of keystrokes if you're not particular about what ends up in $v, since s/// operates on $_:

my $v;
my %myhash = map { $v = $_; s/_/ /g; $v, $_ } @elements;
[download]

Is there a way to pare it down more? (The Monastery needs a golf course!)

Update: As I0 points out, this also modifies @elements, which may not be what you want.

johannz's for construct is looking the most elegant to me now.