#!/usr/bin/perl
use strict;
use warnings;

my %a = ('a' => 1, 'b' => 2);

if (exists $a{'c'}) {
    print "exists before\n";
} else {
    print "not exists before\n";
}

if (defined $a{'c'}) {
    print "defined\n";
} else {
    print "not defined\n";
}

if (exists $a{'c'}) {
    print "exists after\n";
} else {
    print "not exists after\n";
}

for my $i(keys(%a)) {
    print $i . "\n";
}