tests: be really robust to Perl missing

My previous tests (with ./configure PERL=false) have been fooled by
configure, that managed to find perl anyway.  This time, I ran this on
a Fedora in Docker, without Perl.

* tests/calc.at, tests/diagnostics.at, tests/headers.at,
* tests/input.at, tests/local.at, tests/named-refs.at,
* tests/output.at, tests/regression.at, tests/skeletons.at,
* tests/synclines.at, tests/torture.at: Don't require Perl.

tests/headers.at

@@ -314,7 +314,7 @@ AT_TEST([x9], [%locations %code requires {#include "location.hh"} %define api.lo
 #   Ignore comments, YYChar (template parameter), YYPUSH_MORE(_DEFINED)?
 #   (constant definition), YY_\w+_INCLUDED (header guards).
 #   YYDEBUG (not renamed) can be read, but not changed.
-AT_CHECK([[$PERL -n -0777 -e '
+AT_CHECK([["$PERL" -n -0777 -e '
   s{/\*.*?\*/}{}gs;
   s{//.*}{}g;
   s{\b((defined|if)\ YYDEBUG
@@ -347,7 +347,7 @@ AT_CHECK([[$PERL -n -0777 -e '
       print "$ARGV: invalid exported yy: $1\n";
     }
   }
-' -- *.hh *.h]])
+' -- *.hh *.h || exit 77]])
 
 # Do this late, so that other checks have been performed.
 AT_SKIP_IF_CANNOT_LINK_C_AND_CXX