It just occurred to me that using a list concat would cause the OP's

print "${X()} ${X()}\n";
[download]

to behave the same as his

print "${X()}${X()}\n";
[download]

Faster AND fewer surprises :) (Or is it su-prises?)

Of course, the OP will still be surprised by the difference in

$ perl -E'my $x=5; sub X { \(++$x) }  say ${ X() } +     ${ X() };'
14

$ perl -E'my $x=5; sub X { \(++$x) }  say ${ X() } + 0 + ${ X() };'
13
[download]