Porting/sync-with-cpan: more preflight checks for --tarball

If the file supplied doesn't exist (or names a directory), we'll get a
failure from Archive::Tar after having already made some changes.

1 files changed, 2 insertions, 0 deletions

diff --git a/Porting/sync-with-cpan b/Porting/sync-with-cpan
index 13ba07a310..ac861da4c0 100755
--- a/Porting/sync-with-cpan
+++ b/Porting/sync-with-cpan
@@ -270,6 +270,8 @@ if ($cpan_mod =~ /-/ && $cpan_mod !~ /::/) {
 my $new_file;
 my $new_version;
 if (defined $tarball) {
+    die "Tarball $tarball does not exist\n" if !-e $tarball;
+    die "Tarball $tarball is not a plain file\n" if !-f _;
     $new_file     = $tarball;
     $new_version  = $version // ($new_file =~ /-([0-9._]+(?:-TRIAL[0-9]*)?)\.tar\.gz/) [0];
 }