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/backend/utils/mb/Unicode/UCS_to_BIG5.pl

@@ -25,6 +25,8 @@
 #		 # and Unicode name (not used in this script)
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_BIG5.pl';

src/backend/utils/mb/Unicode/UCS_to_EUC_CN.pl

@@ -14,6 +14,8 @@
 # and the "b" field is the hex byte sequence for GB18030
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_EUC_CN.pl';

src/backend/utils/mb/Unicode/UCS_to_EUC_JIS_2004.pl

@@ -8,6 +8,8 @@
 # "euc-jis-2004-std.txt" (http://x0213.org)
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_EUC_JIS_2004.pl';

src/backend/utils/mb/Unicode/UCS_to_EUC_JP.pl

@@ -12,6 +12,8 @@
 # organization's ftp site.
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_EUC_JP.pl';

src/backend/utils/mb/Unicode/UCS_to_EUC_KR.pl

@@ -17,6 +17,8 @@
 #		 # and Unicode name (not used in this script)
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_EUC_KR.pl';

src/backend/utils/mb/Unicode/UCS_to_EUC_TW.pl

@@ -18,6 +18,8 @@
 #		 # and Unicode name (not used in this script)
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_EUC_TW.pl';

src/backend/utils/mb/Unicode/UCS_to_GB18030.pl

@@ -14,6 +14,8 @@
 # and the "b" field is the hex byte sequence for GB18030
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_GB18030.pl';

src/backend/utils/mb/Unicode/UCS_to_JOHAB.pl

@@ -16,6 +16,8 @@
 #		 # and Unicode name (not used in this script)
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_JOHAB.pl';

src/backend/utils/mb/Unicode/UCS_to_SHIFT_JIS_2004.pl

@@ -8,6 +8,8 @@
 # "sjis-0213-2004-std.txt" (http://x0213.org)
 
 use strict;
+use warnings;
+
 use convutils;
 
 # first generate UTF-8 --> SHIFT_JIS_2004 table

src/backend/utils/mb/Unicode/UCS_to_SJIS.pl

@@ -11,6 +11,8 @@
 # ftp site.
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_SJIS.pl';

src/backend/utils/mb/Unicode/UCS_to_UHC.pl

@@ -14,6 +14,8 @@
 # and the "b" field is the hex byte sequence for UHC
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_UHC.pl';

src/backend/utils/mb/Unicode/UCS_to_most.pl

@@ -16,6 +16,8 @@
 #		 # and Unicode name (not used in this script)
 
 use strict;
+use warnings;
+
 use convutils;
 
 my $this_script = 'src/backend/utils/mb/Unicode/UCS_to_most.pl';

src/backend/utils/mb/Unicode/convutils.pm

@@ -6,6 +6,7 @@
 package convutils;
 
 use strict;
+use warnings;
 
 use Carp;
 use Exporter 'import';