You could inspect what the (non-blocking) waitpid returns. If it's the $pid, the process could be reaped successfully, which means it was no longer running.

In your case here, you're calling the waitpid just a tad too early, so the preceding kill TERM hasn't been delivered/succeeded yet.  And as the waitpid $pid => WNOHANG couldn't yet reap at that moment, the process still exists as a zombie at the time you call kill 0  (like in your original case).

Compare:

Child reacts to SIGTERM:

#!/usr/bin/perl -wl
use strict;
use POSIX;

my $pid = fork;
if ( $pid ) {
    kill TERM => $pid;
    # wait for kill TERM to take effect
    select undef, undef, undef, 0.01;  
    my $reaped = waitpid $pid => WNOHANG;
    if ($reaped == $pid) {
        print "already gone.";         # <---
    } else {
        print "trying harder...";
        kill 9 => $pid;
    }
} else {
    sleep 3;
}
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Child ignores SIGTERM:

#!/usr/bin/perl -wl
use strict;
use POSIX;

my $pid = fork;
if ( $pid ) {
    # give child some time to set up its $SIG{TERM} handler
    select undef, undef, undef, 0.01;
    kill TERM => $pid;
    select undef, undef, undef, 0.01;
    my $reaped = waitpid $pid => WNOHANG;
    if ($reaped == $pid) {
        print "already gone.";
    } else {
        print "trying harder...";      # <---
        kill 9 => $pid;
    }
} else {
    $SIG{TERM} = 'IGNORE';
    sleep 3;
}
[download]