Unlike package variables, Perl can have multiple lexical (my) variables with the same name in existence at the same time (Upd: so such a lookup mechanism wouldn't work ).

Nested scopes.

my $x;
{
   my $x;
   ...
}
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Closures.

{ my $x; sub f { ++$x } }
{ my $x; sub g { ++$x } }
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Recursion.

sub f {
   my $x;
   ...
   f();
   ...
}
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Each exist as a different pad entry on the pad of the function that encapsulates it.