Fix pl/perl test case so it will still work under Perl 5.36.

Perl 5.36 has reclassified the warning condition that this test
case used, so that the expected error fails to appear.  Tweak
the test so it instead exercises a case that's handled the same
way in all Perl versions of interest.

This appears to meet our standards for back-patching into
out-of-support branches: it changes no user-visible behavior
but enables testing of old branches with newer tools.
Hence, back-patch as far as 9.2.

Dagfinn Ilmari Mannsåker, per report from Jitka Plesníková.

Discussion: https://postgr.es/m/564579.1654093326@sss.pgh.pa.us

src/pl/plperl/expected/plperl.out

@@ -724,9 +724,9 @@ DO $do$ use strict; my $name = "foo"; my $ref = $$name; $do$ LANGUAGE plperl;
 ERROR:  Can't use string ("foo") as a SCALAR ref while "strict refs" in use at line 1.
 CONTEXT:  PL/Perl anonymous code block
 -- check that we can "use warnings" (in this case to turn a warn into an error)
--- yields "ERROR:  Useless use of sort in scalar context."
-DO $do$ use warnings FATAL => qw(void) ; my @y; my $x = sort @y; 1; $do$ LANGUAGE plperl;
-ERROR:  Useless use of sort in scalar context at line 1.
+-- yields "ERROR:  Useless use of sort in void context."
+DO $do$ use warnings FATAL => qw(void) ; my @y; sort @y; 1; $do$ LANGUAGE plperl;
+ERROR:  Useless use of sort in void context at line 1.
 CONTEXT:  PL/Perl anonymous code block
 -- make sure functions marked as VOID without an explicit return work
 CREATE OR REPLACE FUNCTION myfuncs() RETURNS void AS $$

src/pl/plperl/sql/plperl.sql

@@ -469,8 +469,8 @@ DO $$ use blib; $$ LANGUAGE plperl;
 DO $do$ use strict; my $name = "foo"; my $ref = $$name; $do$ LANGUAGE plperl;
 
 -- check that we can "use warnings" (in this case to turn a warn into an error)
--- yields "ERROR:  Useless use of sort in scalar context."
-DO $do$ use warnings FATAL => qw(void) ; my @y; my $x = sort @y; 1; $do$ LANGUAGE plperl;
+-- yields "ERROR:  Useless use of sort in void context."
+DO $do$ use warnings FATAL => qw(void) ; my @y; sort @y; 1; $do$ LANGUAGE plperl;
 
 -- make sure functions marked as VOID without an explicit return work
 CREATE OR REPLACE FUNCTION myfuncs() RETURNS void AS $$