I wonder if you could use vec instead. I don't think you would need to initialise the string then as it would, I think, be automatically extended at need. I think it also stores bits in the same order as pack q{N}, ...

knoppix@Microknoppix:~$ perl -E '
> vec( $str, 1, 32 ) = 0xfffff;
> say unpack q{b*}, $str;
> vec( $str, 3, 32 ) = 0xf;
> say unpack q{b*}, $str;' 
0000000000000000000000000000000000000000111100001111111111111111
0000000000000000000000000000000000000000111100001111111111111111000000
+0000000000000000000000000000000000000000000000000011110000
knoppix@Microknoppix:~$ perl -E '
> vec( $str, 0, 32 ) = 0xfffff;
> say unpack q{b*}, $str;
> say unpack q{b*}, pack q{N}, 0xfffff;'
00000000111100001111111111111111
00000000111100001111111111111111
knoppix@Microknoppix:~$
[download]

I hope this is useful.