Something as simple as this could be all you need (untested): 

use threads;
use threads::shared;
use DBI;

my $dbh = DBI->connect( );

## Do stuff

my $done :shared = 0;

async {
    $dbh->ping while !$done and sleep 300; ## Adjust frequency to suit
}->detach;

## system 'yourLongRunningCmd';

$done = 1; sleep 1; ## Thread disappears

## The rest of your script

[download]

but you'll have to see if pinging from another thread keeps the connection alive on your system.

Alternatively, if the command runs so long, why not just disconnect from the DB and re-establish a connection when the command is finished?

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

In reply to Re: multiprocessing in perl by BrowserUk
in thread multiprocessing in perl by shijumic

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