You can start a new thread that reads from stdin, and just share the variable $cnt to that thread,

use strict;

use threads;
use threads::shared;

my $cnt:shared = 0;

my $t = threads->new(\&get_input);

while(1){
#simulate some task
    ++$cnt;
    sleep 2;
}

$t->join();


sub get_input{
    while(1){
        my $key = <>;
        chomp $key;
        if($key eq " "){
            print "$cnt\n";
        }
    }
}
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