The other difference is that:

my (@blah) = @_;
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basically copies all parameters passed to the array from @_ to @blah. It leaves @_ intact (for other processing if required).

Whereas:

my $blah = shift;
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will REMOVE the first element of the parameter array @_ and place it in the $blah scalar. 'shift'ing will shorten the @_ array. This, as gaal mentioned, modifies @_.