scripts: enable strict warnings in Perl where missing, fix fallouts

- add 'use warnings' and 'use strict' where missing from Perl scripts.
- fix 'Use of uninitialized value'.
- fix missing declarations.
- test1140.pl: fix 'Possible precedence issue with control flow operator'.
- fix other misc issues.

Most actual errors found during this PR were fixed and merged via
separate PRs.

Likely there are remaining warnings not found and fixed in this PR.

Closes #17877

.github/scripts/cmp-config.pl

@@ -23,6 +23,9 @@
 #
 ###########################################################################
 
+use strict;
+use warnings;
+
 my $autotools = $ARGV[0];
 my $cmake = $ARGV[1];