update - more complete code ( i can't seem to edit the top-level node)::

my %service_labels = ( display_external_host => 'ExternalHost',
                       display_internal_host => 'InternalHost',
                       display_virtual_host => 'VirtualHost',
                      );
my $subset = [ 'ExternalHost' ];

$cgi->checkbox_group( -name => 'FOO',
            -values => $subset,
            -labels => \%service_labels
            );
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prints out this HTML:

<INPUT TYPE="checkbox" NAME="FOO" VALUE="ExternalHost">ExternalHost
<INPUT TYPE="checkbox" NAME="FOO" VALUE="VirtualHost">VirtualHost
<INPUT TYPE="checkbox" NAME="FOO" VALUE="InternalHost">InternalHost
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what I expected ( or what I want );

<INPUT TYPE="checkbox" NAME="FOO" VALUE="display_external_host">Extern
+alHost
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which is what i'd get if i were to use  [ keys \%service_labels ] as my -values argument