tests: use $PERL instead of perl

* tests/atlocal.in (PERL): New.
Sort.
* tests/calc.at, tests/input.at, tests/local.at, tests/regression.at,
* tests/skeletons.at, tests/synclines.at, tests/torture.at: here.

tests/local.at

@@ -38,7 +38,7 @@ m4_define([m4_null_if],
 # Expect COUNT matches of the PERL-REGEXP in FILE.  The file is
 # taken in "slurp" mode, i.e., one can match end-of-lines.
 m4_define([AT_MATCHES_CHECK],
-[AT_CHECK([perl -0777 -ne '
+[AT_CHECK([$PERL -0777 -ne '
 my $count = 0;
 s{$2}{ ++$count; "" }gem;
 printf "$count\n";' $1], [0], [$3