Hmm, are you sure you want 256x1024**2 in that substr? I think this is increasing the number beyond what you intended , which doesn't matter with substr because you're replacing all that trailing string with one char, but does matter with vec because nothing is cut-off

Adjusted for my machine I run

A perl -e"$x=chr(0);$x x=5*1024**2; <>;$r=\substr($x, 2*1024**2); <>;vec($$r,2*1024**2-1,1)=1; <>"

B perl -e"$x=chr(0);$x x=5*1024**2; <>;$r=\substr($x, 2*1024**2); <>;substr($$r,2*1024**2-1,1)='x'; <>"

And I consistently get results like

A	B
7,760 k 7,776 k 10,876 k	7,760 k 7,776 k 10,880 k