I don't use CGI.pm to generate HTML, but I think you can add -value to your args. Something like this:

sub getInvest 
{ 
    my $investID = shift || return {};
    # grab row from database with fethrow_hashref()
    return \%invest;
}

sub defaultNewInvest
{
   my $q = shift;
 
   my $fillIn = getInvest($q->param('investID'));

   # ...

   $q->td( ["Title:", $q->textfield(-name=>"invest_title", -size=>40, 
+-value=>$fillIn->{'invest_title'} )]) ]);
}
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Using a templating solution like HTML::Template is a good way to handle this type of thing, because you can usually just pass the hash you get from your database call right into the template's param() method.