In general, you can use "kill" with signal number zero to check on the existence of a process. See 'man perlipc' for details. Here's code:

use strict;

MAIN:
{
    my $pid = $ARGV[0];

    if (kill 0 => $pid) {

        print STDOUT "Process '$pid' exists\n";

    }
    else {
        print STDOUT "Process '$pid' does not exist\n";
    }

}
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