Use perl warnings pragma consistently

We've had a mixture of the warnings pragma, the -w switch on the shebang
line, and no warnings at all. This patch removes the -w swicth and add
the warnings pragma to all perl sources missing it. It raises the
severity of the TestingAndDebugging::RequireUseWarnings  perlcritic
policy to level 5, so that we catch any future violations.

Discussion: https://postgr.es/m/20200412074245.GB623763@rfd.leadboat.com

src/tools/perlcheck/perlcriticrc

@@ -19,6 +19,10 @@ verbose = %f: %m at line %l, column %c.  %e.  ([%p] Severity: %s)\n
 # allow octal constants with leading zeros
 [-ValuesAndExpressions::ProhibitLeadingZeros]
 
+# insist on use of the warnings pragma
+[TestingAndDebugging::RequireUseWarnings]
+severity = 5
+
 # for now raise severity of this to level 5
 [Subroutines::RequireFinalReturn]
 severity = 5