Question

I am newbie learning perl. The question I have is how do I make sure that if the mine is not set, after the while, I need to send an error or exception?

while (my ($a, $b) = each %$him) {
    if (($c->{$d}) eq $a) {
        $mine = $b;
    }
}

Here I have to return error if $mine is not set.

Answer 1

That whole while loops is unnecessary. You just need

die if !exists($him->{ $c->{$d} });
my $mine = $him->{ $c->{$d} };

You might prefer

# If doesn't exist or isn't defined
die if !defined($him->{ $c->{$d} });
my $mine = $him->{ $c->{$d} };

or

# If doesn't exist, isn't defined, or is false.
die if !defined($him->{ $c->{$d} });
my $mine = $him->{ $c->{$d} };