Thank you, John!  Yes, that worked for me, the function looks like this:

$c->add_dataset_trigger( 'eprint', EPrints::Const::EP_TRIGGER_BEFORE_COMMIT, sub

{

    my( %args ) = @_;

    my( $session, $eprint, $changed ) = @args{qw( repository dataobj changed )};

   if( exists $changed->{'field_of_interest'} )

    {

        my $old_val = $changed->{'field_of_interest'};

        my $new_val = $eprint->get_value( 'field_of_interest');

                        my $user = $eprint->get_user();

                        my $username = EPrints::Utils::tree_to_utf8( $user->render_description() );

        # generate an email alert

                        if ($old_val eq 'FALSE' && $new_val eq 'TRUE') {

                                    print STDERR "field_of_interest Changed from ".$old_val." ".$new_val." SEND NOTIFICATION EMAIL";

                                    my $message = $session->make_element( "p" );

                                    $message->appendChild( $session->make_text("Hello " . $username.","));

                                    $message->appendChild($session->make_element( "br" ));

                                    $message->appendChild($session->make_element( "br" ));

                                    my $title = $session->make_element( "strong" );

                                    $title->appendChild($session->make_text($eprint->get_value("title")));

                                    $message->appendChild($session->make_text( "This email is to let you know that the following item you deposited: "));

                                    $message->appendChild($title);

                                    $message->appendChild($session->make_text(" (".$eprint->get_url().")"));

                                    $message->appendChild($session->make_text(" has changed in a specific way."));

                                    my $ok = $user->mail("field_change_phrase",$message);

                        }

    }

}, priority => 73);

This works!  The only thing is, I would like the action to register in the history for the item, any idea how would I do that?

Tomasz

From: John Salter <J.Salter@leeds.ac.uk>
Sent: August 3, 2022 5:37 PM
To: eprints-tech@ecs.soton.ac.uk; Tomasz Neugebauer <Tomasz.Neugebauer@concordia.ca>
Subject: RE: [EP-tech] email notification on a specific metadata field value change

Attention This email originates from outside the concordia.ca domain. // Ce courriel provient de l'exterieur du domaine de concordia.ca

This sounds like a job for a commit trigger.
There is a 'changed' hash - which has the old field values in it (the EPrint object already has the newer values in it).

You can check whether the field you are interested in is a key in the changed hash, and use that as a basis for sending an email.

NB The example below may not be syntactically correct. It's lashed together late at night (for me anyway), but I think you'll get the idea. Happy to answer questions about it in (my) morning!

Cheers,

John

$c->add_dataset_trigger( 'eprint', EPrints::Const::EP_TRIGGER_BEFORE_COMMIT, sub

{

    my( %args ) = @_;

    my( $repo, $user, $changed ) = @args{qw( repository dataobj changed )};

   if( exists $changed->{'the_field_youre_interested_in'} )

    {

        my $old_val = $changed->{'the_field_youre_interested_in'};

        my $new_val = $eprint->get_value( 'the_field_youre_interested_in' );

        # generate an email alert

    }

}, priority => 73);