thank you very much for your suggestion.
yours and Browser_Uk were the same ideas actually
so I have documented myself a bit and written a daemon
which is sitting and monitoring for changes in files.
the following is what I cam up with(teste and works excellent):

#!/usr/bin/perl 
#this lightweight daemon will run in the background and
#for each new file that is modified and closed it will re-hash it and 
+update
#its entry in the database
use strict;
use warnings;
#
#bugs encountered:
#
#1)does not update as expected in database the new sha1
#2)$mtime seems to be very VERY different from DateTime->now which is 
+weird
#3)bug found in SHA1db find_or_update not finding correctly if there a
+re any files in the db with
#that name
#
#
use Linux::Inotify2;
use Data::Dumper;
use DateTime;
use SHA1db;
use YAML qw/LoadFile/;
$|=1;
SHA1db->connect();
my $inotify = new Linux::Inotify2();

my $config_path = 'config.yml';
my $config = LoadFile($config_path);

for (map {$_->{path}} @{ $config->{directories} }) {
  print "tracking $_\n";
  $inotify->watch($_, IN_ALL_EVENTS);
}

while () {
    my @events = $inotify->read;
    unless (@events > 0) {
        print "read error: $!";
        last ;
    };
    for(@events) {
        next unless($_->mask & IN_CLOSE_WRITE);
        my $mtime =( lstat($_->fullname) )[9];
        next unless $mtime;
        printf "updating checksum_db for file %s modified now: %s\n",$
+_->fullname,DateTime->from_epoch(epoch=>$mtime);
        #we should add a check to see if this file passes the regex
        #filter & link filter & dir filter & size filter
        unless(SHA1db::find_or_update($_->fullname,$mtime)) {
            printf "not found in db,adding...\n";
            SHA1db::add_to_db(SHA1db::file2sha1($_->fullname),$mtime,-
+s _ ,$_->fullname);
            #add it to db
        };
    };
};

print "daemon stopped";
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