With recent versions of threads, you can install per-thread signal handlers. Using these in conjunction with a signal handler in your main thread, you can forward process signals to your threads and have each one deal with it appropriate to it's context. In this case, killing the current child process.

This is necessarily untested code, but it should serve to demonstrate the idea:

#! perl -slw
use strict;
use threads;
use Thread::Queue;
use LWP::Simple;

our $N ||= 2;

my $Q = new Thread::Queue;

sub dbDump {
    my $pid;

    ## Add per-thread signal handlers closing over the $pid
    @SIG{ qw[TERM ALRM INT HUP] } = ( sub{ kill 9, $pid } ) x 4;

    my $opts = '-q --single-transaction --complete-insert';

    while( my $dbinfo = $Q->dequeue ) {
        my( $dbname ) = split ' ', $dbinfo, 1;
        my $outfile = $dbname . localtime() . '.dmp';
        my $pid = open my $cmd,
            "mysqldump $opts $dbinfo --results-file=$outfile |"
            or die $!;
    }
}

my @pool = map &async( \&dbDump ), 1 .. $N;

## Add main thread sig handlers to relay process signals to threads
@SIG{ qw[TERM ALRM INT HUP] } = ( sub{ $_->kill( 'TERM' ) for @pool } 
+) x 4;

for my $db (keys %{$db_ds}) {
    if (!@{$db_ds->{$db}} && !$opts{database}) {
        ### Our data stuctur says there is no native mysql tables.
        ### So we skip this database.
        next;
    }
    $dbh->do(qq{use $db});
    ### The table list in our data structure is empty.  The database
    ### option has been passed, so look them up.
    if (!@{$db_ds->{$db}}) {
        $queries{get_table_names}->execute();
        while (my $rec = $queries{get_table_names}->fetchrow_hashref()
+) {
            push @{$db_ds->{$db}}, $rec->{Name};
        }
        debug(qq{Got table list for $db\n});
    }

    ### This block makes sure we can actually access the tables.
    my @valid_tables;
    for my $table (@{$db_ds->{$db}}) {
        $queries{verify_table_name}->execute($table);
        if ($queries{verify_table_name}->rows()) {
            my ($rec) = $queries{verify_table_name}->fetchrow_array();
            push @valid_tables, $rec;
        } else {
            debug(qq{There was a problem finding database/table $db $t
+able\n});
        }
    }
    $Q->enqueue( join ' ', $db, @valid_tables );
}

$Q->enqueue( (undef) x $N );

$_->join for @pool;
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