use Module qw(somesub);
[download]

is equivalent to

BEGIN {
   require Module;
   Module->import('somesub') if Module->can('import');
}
[download]

You've prooved that require Module; works, and there's no reason to believe if Module->can('import') fails, so we're left with Module->import('somesub'). My bet is that importing works, but it's importing to the wrong namespace. Look into which namespace is used for once vs init. If the following work, my hypothesis is confirmed:

<%once>
# warn("once: " . __PACKAGE__ . "\n");

require Module;
# or:      use Module ();
# but not: use Module;
# An empty list disables import,
# but ommiting the list does not.
</%once>
...
...
...
<%init>
# warn("init: " . __PACKAGE__ . "\n");

Module->import(qw(somesub));

... &somesub(xxx);

</%init>
[download]