Go ahead and add use diagnostics; to see

$ perl -Mdiagnostics -e " $foo = 1; "
Name "main::foo" used only once: possible typo at -e line 1 (#1)
    (W once) Typographical errors often show up as unique variable nam
+es.
    If you had a good reason for having a unique name, then just menti
+on it
    again somehow to suppress the message.  The our declaration is
    provided for this purpose.

    NOTE: This warning detects symbols that have been used only once s
+o $c, @c,
    %c, *c, &c, sub c{}, c(), and c (the filehandle or format) are con
+sidered
    the same; if a program uses $c only once but also uses any of the 
+others it
    will not trigger this warning.
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warnings are useful, but adding strict is better. Read this if you want to cut your development time in half! explains how/why to use them.