$ 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 names.
    If you had a good reason for having a unique name, then just mention 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 so $c, @c,
    %c, *c, &c, sub c{}, c(), and c (the filehandle or format) are considered
    the same; if a program uses $c only once but also uses any of the others it
    will not trigger this warning.

##</code><code>##

$ perl -e " use diagnostics; my $c = $c; my $c = $c; "
"my" variable $c masks earlier declaration in same scope at -e line 1 (#1)
    (W misc) A "my", "our" or "state" variable has been redeclared in the
    current scope or statement, effectively eliminating all access to the
    previous instance.  This is almost always a typographical error.  Note
    that the earlier variable will still exist until the end of the scope
    or until all closure referents to it are destroyed.

Name "main::c" used only once: possible typo at -e line 1 (#2)
    (W once) Typographical errors often show up as unique variable names.
    If you had a good reason for having a unique name, then just mention 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 so $c, @c,
    %c, *c, &c, sub c{}, c(), and c (the filehandle or format) are considered
    the same; if a program uses $c only once but also uses any of the others it
    will not trigger this warning.