$dat0 = 'a.txt';
open( DAT, "$dat0" ) || die ("Could not open file!");
@all  = <DAT>;
# I'm not enven sure what perl will do when you open the same file handle
# twice without closing, I would assume it would close the first for you,
# but it's just bad.
open( DAT, ">$dat0" ) || die;
# You never write back to @all so you are doing nothing.
print DAT @all;
close(DAT);