solver: drop warnings, modernize use v5.10.1, use SEEK_SET
With Perl upstream preparing to deprecate things, we'll move
towards only enabling warnings during development via shebang
and stop enabling them via "use".
We'll also favor "use v5.10.1" over the Perl 5.6-compatible "use
5.010_001", since our code base never worked on 5.6.
Finally, were also importing SEEK_SET without using it, just use it
for readability since we can't avoid loading Fcntl in other
places and it'll get constant-folded, anyways.