This is untested code, and given the lack of detail in your question, assumes a lot of things.

The basic idea is to start a thread that monitors a shared time value and if we ever reach that time, issues the keep alive command and resets the timer.

The class wraps the Telnet object (by composition) and overrides the cmd() method to ensure the timer gets reset before the cmd is issued. It also adds a DESTROY() method to clean up the thread when the session ends.

Adding accessors and error handling is left as an exercise.

package My::Telnet;
use threads::shared;
use Net::Telnet;

## Thread to keep session alive
sub _keepAlive {
    my( $tn, $done, $keepAlive ) = @_;
    
    ## Die when signalled
    ## Wake up once per second
    while( not $done and sleep 1 ) {
        
        ## If it's time, send the keep alive cmd.
        if( time() >= $keepAlive ) {
            my @output = $tn->cmd(
                ## Keep alive command goes here
            );
            
            ## and reset the timeout
            $keepAlive = time() + 30;
        }
    }
}

sub new {
    my( $class, $host, $port ) = @_;
    
    ## Create the session
    my $tn = Net::Telnet->new(
        host => $host,
        port => port,
        errmode => sub{ warn "Error: $_[ 0 ]"; },
    ) or die $!;
    
    ## Some shared vars for communication
    my $done :shared = 0;
    my $keepAlive :shared = time() + 30;

    ## Start the thread.
    my $thread = threads->create( 
        \&_keepAlive, $tn, $done, $keepAlive
    } or die $!;

    ## build the object and return it
    bless {
        done => \$done,
        telnet => $tn,
        host => $host,
        port => $port,
        thread => $thread,
        keepAlive => \$keepAlive;
    }, $class;
}

## resets the timer to the time supplied 
## or now + 30 seconds
sub resetKeepAlive {
    my( $self, $timeout ) = @_;
    $$self->{ keepAlive } } = $timeout || ( time() + 30 );
}

## Issues a command
## Resetting the timer to prevent the next keep alive.
sub cmd {
    my( $self, %cmdNargs ) = @_;
    
    ## reset the timer *before* issuing the command
    $self->resetKeepAlive;
    
    ## Do the command and return the output
    return $self->{ telnet }->cmd( %cmdNargs )
}

## Clean up by telling the thread to die
## waiting for it to do so
## and closing the connection.
sub DESTROY {
    my( $self ) = @_;
    $$self->{ done } = 1;
    $self->{ thread }->join;
    close $self->{ telnet };
    return;
}
[download]