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 $ perl -e " use diagnostics; my $c = $c; "
Name "main::c" used only once: possible typo at -e line 1 (#1)
    (W once) Typographical errors often show up as unique variable nam
+es.
...
    %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.

$ perl -e " use diagnostics; my $c = $c; my $c = $c; "
"my" variable $c masks earlier declaration in same scope at -e line 1 
+(#1)
...
    will not trigger this warning.