Now that I think about it, I'd it's all about timing. Consider

perl -e"exec $^X,qw[ -e sleep(1);warn(666)];die 55" && echo BOO
perl -e'exec $^X,qw[ -e sleep(1);warn(666)];die 55' && echo BOO
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and vary the sleep duration (0 and up). On Win2000 I get

E:\>perl -e"exec $^X,qw[ -e sleep(0);warn(666)];die 55" && echo BOO
666 at -e line 1.
BOO

E:\>perl -e"exec $^X,qw[ -e sleep(1);warn(666)];die 55" && echo BOO
BOO

E:\>666 at -e line 1.
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and on debian with bash I get

crazyinsomniac@perlmonk:~$ perl -e'exec $^X,qw[ -e sleep(1);warn(666)]
+;die 55' && echo BOO
666 at -e line 1.
BOO
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regardless of the sleep duration (the sleep just delays the output). The bash might be fixing the results with some kind of buffering but I can't tell.