OK, here is the expanded version of the original OP's oneliner, following tye's explanation:

cat fatals.pl
#!/usr/bin/perl

use strict;
use warnings FATAL => 'all';

my $cmd = shift || '_bad_exe_ a b c';
print "I'm parent with id $$\n";

eval {
    my $pid = open CMD, "-|";
    if ($pid) {
        print "I'm forking a child with id $pid\n";
        print while <CMD>;
    }
    else {
        #no warnings; # get rid of warnings on "Can't exec:..."
        print "[$$] I'm child, about to exec [$cmd]\n";
        exec $cmd;
    }
};

die "[$$] ERROR: $@" if $@;
print "[$$]HOW DID I GET HERE?\n";
[download]

Run with correct command (just as comparisson):

$ perl fatals.pl ls
I'm parent with id 6238
I'm forking a child with id 6239
[6239] I'm child, about to exec [ls]
642663.pl
642676.pl
642682.pl
644761.pl
[download]

No problem there. Now, run with no args (default command):

$ perl fatals.pl
I'm parent with id 6240
[6241] ERROR: Can't exec "_bad_exe_": No such file or directory at fat
+als.pl line 18.
I'm forking a child with id 6241
[6241] I'm child, about to exec [_bad_exe_ a b c]
[6240]HOW DID I GET HERE?
[download]

So I guess, it's the child that triggers the "Can't exec ..." warning that gets "upgraded" into fatal error (due to FATAL), trapped in $@, and dies. The child dies, the parent doesn't, that's HOW DID I GET HERE, I suppose. OTOH, if the no warnings; is uncommented, then well, no "Can't exec..." warning, no $@ trapping, and the child lives pretending nothing happens, yet covering up by asking HOW DID I GET HERE.