What's the difference between various $SIG{CHLD} values?

Question

What is the difference between these settings?

$SIG{CHLD} = \'IGNORE\'  
$SIG{CHLD} = \'DEFAULT\'  
$SIG{CHLD} = \'\'  
$SIG{CHLD} = undef

Answer 1

$SIG{CHLD} = 'IGNORE'

This is passed to the OS as SIG_IGN, which according to the OS docs, makes the child processes terminate without becoming zombies (or reporting their exit code to the parent).

$SIG{CHLD} = 'DEFAULT'
$SIG{CHLD} = ''
$SIG{CHLD} = undef

These are all the same at the OS level (they will call signal() with SIG_DFL). However some perl packages, notably AnyEvent::child, will not work when $SIG{CHLD} is set to 'DEFAULT'.