#!/usr/bin/perl -l
use strict;
use warnings;
use PadWalker ":all";

sub foo {
   my $x;
   for ($x) {
      my $x;
      bar(\$_);
   }
}

sub bar {
    print var_name(1, $_[0]);  # works
    my $vars = peek_my(1);
    print grep $vars->{$_} == $_[0], keys %$vars;  # doesn't work unless 2nd my $x is commented out
}

foo();