Use the source, Luke. The module's code isn't particularly hard to understand. It is clear that you can't change @_ (only elements of @_) in a pre-wrapper and the example with splice in the docs is clearly wrong.

You can wrap a routine yourself much more flexibly and less magicly with rather simple code:

sub some_sub {
    print "(@_)\n";
}

my $orig= \&some_sub;
*some_sub= sub {
    $orig->( 'newArg', @_ );
};

some_sub( "test", "ing" );
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produces:

(newArg test ing)
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Make sense?