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

sub myjoin {
    my ($sep, $href, @keys) = @_;
    my @exists = grep { exists($href->{$_}) } @keys;
    return join $sep, @exists;
}

my %hash = (key1 => 'a');
my $s1 = join( ':', @hash{qw(key)} );
mydump( \%hash );
my $s2 = myjoin( ':', \%hash, qw/key1 key2 key3/ );
mydump( \%hash );