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]

In reply to Re^3: Optimising concat (was Order of evaluation/interpolation of references) by ikegami
in thread Order of evaluation/interpolation of references by Anonymous Monk

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