$_ = qw(nothing nothing)
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Translates to:
1. Under 5.005

@_ = qw(nothing nothing)
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2. Under 5.6
No effect on @_;
Resulting
call for &subroutine
1. Under 5.005
prints "..nothing..nothing"
2. Under 5.6
prints "..alpha ..omega"

Try:

print qw(1 2 3)[0]
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under 5.005 and 5.6.