use Data::Dumper;
sub mydump($) {
    print Data::Dumper->Dump( [ $_[0] ], ['*hash'] );
}

sub myjoin {
    return unless exists($_[1]->{$_[2]});
    return join $_[0], $_[1]->{$_[2]};
}

my %hash;
my $s1 = join( ':', @hash{qw(key)} );
mydump( \%hash );
my $s2 = myjoin( ':', \%hash, 'key' );
mydump( \%hash );