sub check_foo {
  if (exists $foo{bar}) {
    return $foo{bar};
  }else{ 
    die "CODE[$the_mysterious_errorcode]\n";   
  }
}
# instead of my $value = check_foo();
my $value = eval { check_foo() };

### see if there was an error
if( $@ ){
  if( $@ =~ /CODE\[(.+)\]/ ){
    my $code = $1;
  }
}

### if no problem with eval, $value is set to $foo{bar}