use diagnostics;
use warnings;
use strict;
use Data::Dumper;

my %h;
$h{ 'a' }{ 'b' } = 5;

my $b_code =<<'CODE';

sub tryme_b
{
   my $foo = shift;
   return ${ $foo }{ 'a' }{ 'b' };
}

CODE

my $c = "$b_code; tryme_b( \%h );";
my $ans = eval $c or die "[$c] doesn't work:$@";

print "b:$ans,\n";