Some solutions that don't require a special map or grep:

%h_numbers = map {looks_like_number($h{$_}) ? ($_, $h{$_}) : ()} keys 
+%h;  # Or
%h_numbers = map {($_, $h{$_})} grep {looks_like_number($h{$_})} keys 
+%h;

%h_prefixed = map {($_, "p".$h{$_})} keys %h; # Or
%h_prefixed = %h; $_ = "p$_" for values %h_prefixed;

# Both solutions below destroy @a; but you can make a copy first.
while(my($x,$y) = splice @a, 0, 2) {push @a_hashed, {$x, $y}}  # Or
push @a_hashed, {shift @a, shift @a} while @a;
[download]