Well, I've created a small sub:

sub reload {
    delete $INC{'spamassassin.pm'};
    require 'spamassassin.pm';
}
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and it seems to work fine. I got the subs redifined...

prep test shell >reload
Subroutine spamassassin_report redefined at spamassassin.pm line 14.
Subroutine spamassassin_graph redefined at spamassassin.pm line 56.
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If I use do, I get a warning...

prep test shell >reload
Use of uninitialized value in do "file" at ./shell.pl line 46.
Null filename used at ./shell.pl line 46.
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