I think that this behaviour is caused by the fact that Perl has to launch a shell to process a command given in a string form, and the open doesn't fail because shell does launch successfully (it's the command that fails later):

use Test::Simple tests => 3;
ok !open(my $fh, '|-', 'nonexistent program'), 'string form of open fa
+ils';
ok open(my $prince, '|-', 'nonexistent program 2>/dev/null'), 'but suc
+ceeds if shell uses redirection';
ok !open(my $none, '|-', 'nonexistent', 'program'), 'list form of open
+ fails, as it should';
__END__
1..3
ok 1 - string form of open fails
ok 2 - but succeeds if shell uses redirection
ok 3 - list form of open fails, as it should
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But why does string form of open correctly fail in the first test? Perl is able to process a simple command without help of a shell, as strace shows:

$ strace -f -e clone,execve perl -e'open(my $fh, "|-", "nonexistent pr
+ogram")'
execve("/usr/bin/perl", ["perl", "-eopen(my $fh, \"|-\", \"nonexisten"
+...], [/* 45 vars */]) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIG
+CHLD, child_tidptr=0xb74f7a68) = 7792
Process 7792 attached
[pid  7792] execve("/home/aitap/perl5/bin/nonexistent", ["nonexistent"
+, "program"], [/* 45 vars */]) = -1 ENOENT (No such file or directory
+)
[pid  7792] execve("/home/aitap/scripts/nonexistent", ["nonexistent", 
+"program"], [/* 45 vars */]) = -1 ENOENT (No such file or directory)
[pid  7792] execve("/home/aitap/bin/nonexistent", ["nonexistent", "pro
+gram"], [/* 45 vars */]) = -1 ENOENT (No such file or directory)
[pid  7792] execve("/usr/local/bin/nonexistent", ["nonexistent", "prog
+ram"], [/* 45 vars */]) = -1 ENOENT (No such file or directory)
[pid  7792] execve("/usr/bin/nonexistent", ["nonexistent", "program"],
+ [/* 45 vars */]) = -1 ENOENT (No such file or directory)
[pid  7792] execve("/bin/nonexistent", ["nonexistent", "program"], [/*
+ 45 vars */]) = -1 ENOENT (No such file or directory)
[pid  7792] execve("/usr/local/games/nonexistent", ["nonexistent", "pr
+ogram"], [/* 45 vars */]) = -1 ENOENT (No such file or directory)
[pid  7792] execve("/usr/games/nonexistent", ["nonexistent", "program"
+], [/* 45 vars */]) = -1 ENOENT (No such file or directory)
[pid  7792] +++ exited with 1 +++
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=7792, si_uid
+=1000, si_status=1, si_utime=0, si_stime=0} ---
+++ exited with 0 +++
[download]

$ strace -f -e clone,execve perl -e'open(my $fh, "|-", "nonexistent pr
+ogram 2>/dev/null")'
execve("/usr/bin/perl", ["perl", "-eopen(my $fh, \"|-\", \"nonexisten"
+...], [/* 45 vars */]) = 0
clone(Process 7809 attached
child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, 
+child_tidptr=0xb753ea68) = 7809
[pid  7809] execve("/bin/sh", ["sh", "-c", "nonexistent program 2>/dev
+/null"], [/* 45 vars */]) = 0
[pid  7809] +++ exited with 127 +++
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=7809, si_uid
+=1000, si_status=127, si_utime=0, si_stime=0} ---
+++ exited with 0 +++
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