I work around not being able to break inside of a module before the module has finished loading by just modifying the module's source code to add $DB::single = 1; before the line I want the debugger to break at.

You can also use "b load ...". Though, I found that command to be overly picky about how you specify the file name so I patched my perl5db.pl to make it less picky:

...
sub cmd_b_load {
    my $file = shift;
    my @files;

    $file =~ s!::!/!g;   # Added this line
...
    $signal = 1, print $OUT "'$filename' loaded...\n"
#     if $break_on_load{$filename};  # Replaced this line with the fol
+lowing:
      if grep $break_on_load{$_}, $filename =~ m{(?:^|[/\\])(?=(.*))}g
+;
...
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