With threads:

#! perl -slw
use strict;
use threads;
use threads::shared;

my @output :shared;
my $pid :shared; 

my $t = async {
    $pid = open CMD,
        qq[ perl -le"\$|++; print ~~localtime while sleep 1" |]
        or die $!;
    while( <CMD> ) {
        chomp;
        lock @output;
        push @output, $_;
    }
    close CMD;
};

sleep 10; ## do other stuff

for my $out ( do{ lock @output; @output } ) {
    if( $out =~ m[6] ) {
        print $out;
    }
}

kill 9, $pid;
$t->join;
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