Whenever you get a warning or error you don't understand, look it up in perldiag. There are a couple automated ways to do this. The diagnostics module:

$ touch foo.pl
$ perl -we'use diagnostics; require "foo.pl"'
foo.pl did not return a true value at -e line 1 (#1)
    (F) A required (or used) file must return a true value to indicate
+ that
    it compiled correctly and ran its initialization code correctly.  
+It's
    traditional to end such a file with a "1;", though any true value 
+would
    do.  See perlfunc/require.

Uncaught exception from user code:
        foo.pl did not return a true value at -e line 1.
 at -e line 1
[download]

or the splain utility:

$ echo "a.pl did not return a true value at ./require.plx line 7." | s
+plain
a.pl did not return a true value at ./require.plx line 7 (#1)
    (F) A required (or used) file must return a true value to indicate
+ that
    it compiled correctly and ran its initialization code correctly.  
+It's
    traditional to end such a file with a "1;", though any true value 
+would
    do.  See perlfunc/require.
[download]