Looks like a problem 'use strict' would have caught. It can be fixed by just inserting one line into "sub DB" of perl5db.pl:

    my $action;
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Update (late): Broken by:

commit 04e43a21088e2c6b07ebde9e57007350cfd7310b
Author: Daniel S. Lewart <lewart@uiuc.edu>
Date:   Tue May 22 21:18:03 2001 -0500
[download]

specifically:

-    if (($stop,$action) = split(/\0/,$dbline{$line})) {
+    if ($dbline{$line} && (($stop,$action) = split(/\0/,$dbline{$line
+}))) {
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But I'd still fix it by just putting a line before that:

my( $stop, $action );
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There's nothing like fixing a decade-old bug.