perl -we" my $foo = 1; my $foo = 2; "
"my" variable $foo masks earlier declaration in same scope at -e line 1.

$ perl -Mdiagnostics -we" my $foo = 1; my $foo = 2; "
"my" variable $foo 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.