Interestingly enough, taint-checking will make your first example to throw that error as well....

% perl -Te 'push @{ ($_ % 2) ? @odd : @even }, $_ for 1..20'
Bizarre copy of ARRAY in leave at -e line 1.

% perl -e  'push @{ ($_ % 2) ? @odd : @even }, $_ for 1..20'
[no error.....]
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-Blake