Because thats how its supposed to work :) strict, diagnostics, splain...the spirit of strict ...

 $ 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.
    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.
[download]

The $c on the right side of my $c = is the global variable $main::c, and $main::c is not the same as $c

Also

$ 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 sc
+ope
    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 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.
[download]

So its  my $c = $main::c;  my $c = $c; and the second lexical $c takes places of the original lexical $c (my $c) ... so the first my $c is invisible

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