Many people frown on using map and grep in a void context. Both commands are intended for building a list that is returned at the end; if you don't need the returned list from map or grep, it's cleaner to use foreach instead.

my %myhash;
foreach (@elements) {
  my $v = $_;
  $v =~ tr/_/ /;
  $myhash{$_} = $v;
}
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UPDATE: runrig pointed out that the my $v can be moved inside the loop, which didn't occur to me as I was rewriting the map to a foreach. I've made that change now.