build: Wrap perl expression in PERL_LIBDIR assignment

m4/perl.m4

@@ -5,5 +5,7 @@ AC_DEFUN([DPKG_PROG_PERL],
 [AC_ARG_VAR([PERL], [Perl interpreter])dnl
 AC_PATH_PROG([PERL], [perl], [/usr/bin/perl])dnl
 AC_ARG_VAR([PERL_LIBDIR], [Perl library directory])dnl
-PERL_LIBDIR=$($PERL -MConfig -e 'my $r=$Config{vendorlibexp};$r=~s/$Config{vendorprefixexp}/\$(prefix)/; print $r')dnl
+PERL_LIBDIR=$($PERL -MConfig -e 'my $r = $Config{vendorlibexp};
+                                 $r =~ s/$Config{vendorprefixexp}/\$(prefix)/;
+                                 print $r')dnl
 ])# DPKG_PROG_PERL