Forgetting how to use vec

[Aaron Sherman]

I have a small piece of Perl code that does something like this:

for($i=0;$i<length($x);$i++){
vec($y,$i*7,8)=vec($x,$i*8,8);
}

But it doesn't pack 7-bit data from $x into $y the way I thought it
would. Could someone enlighten me on how I'm mis-reading this? What I
really want is to pack the low 7 bits of $x into $y, with no
high-bit-padding.