sub reverse_inplace
{
    for ( my $i = 0; $i < $#_ / 2; $i++ )
    {
        my $j = $#_ - $i;
        @_[$i,$j] = @_[$j,$i]; # swap
    }
}

# try it:

my @a = ( 1 .. 9 );
reverse_inplace( @a );
print "@a\n";