my %foo =
  map { (split /:/, $_, 2)[0, 1] } # Implicit "default" undef.
  qw/
    foo:bar
    baz
    zip:zap:zoom
  /
;

use Data::Dumper;
print Dumper(\%foo);

__END__
$VAR1 = {
          'baz' => undef,
          'foo' => 'bar',
          'zip' => 'zap:zoom'
        };