Localize the handler with a closure, like this:

sub foo {
    my ($a, $b, $c) = @_;
    my %hash = (a => {b => {c => 0}});
    # whatever other variables you want to initialize
    local $SIG{__WARN__} = sub {
        # dump those variables here
    };
    print $hash{$a}->{$b}->{$c};
    # ...
}
[download]

The variables you want to dump will be in scope of the __WARN__ handler and you can do whatever you like with them.

local $_ = "0A72656B636148206C72655020726568746F6E41207473754A";
while(s/..$//) { print chr(hex($&)) }
[download]