I've seen that error before when using DBI and Parallel::ForkManager. The problem arises when a $dbh is inherited from the parent, and several childs try to use the same connection at the same time. I solved this as follow:

    my $fork_manager = new Parallel::ForkManager(10);
    $fork_manager->start and next; # fork

    # Clone parent dbh
    my $dbh_child = $dbh->clone();
    $dbh->{InactiveDestroy} = 1;
    $dbh = $dbh_child;
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So you can try:

sub db_execution {
    ...
    my $dbh = $db->connect( 'students' ) or die $!;
    my $dbh_child = $dbh->clone();
    $dbh->{InactiveDestroy} = 1;
    $dbh = $dbh_child;
    my $sth = $dbh->prepare( $sql ) or die "$!:" . $dbh->errstr;
    $sth->execute or die "$!:" . $sth->errstr;
    ...
}
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Hope this helps