The Perl Cookbook advises being paranoid and always reinstalling your SIG handler as the first step of your signal handler except in the case of SIGCHLD for the following reason:

If you re-install the SIGCHLD handler before wait or waitpid gets its information from the OS's process table you risk the child becoming a zombie. Recomended is the following: Loop in the SIGCHLD handler and only re-install the SIGCHLD handler after the loop is complete (code below taken from Perl Cookbook).

$SIG{CHLD}=\&REAPER;
sub REAPER{
  my $stiff;
  while(($stiff = waitpid(-1,&WNOHANG))>0){
    # do something with $stiff if you want
  }
  $SIG{CHLD}=\&REAPER;
}
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This is necessary because most OSes will not queue up signals so if 2 are sent to your process at aprox the same time your process may behave as if it has received only 1.

This may point to a possible solution to the original problem:

$SIG{CHLD}=\&REAPER;
sub REAPER{
  $SIG{CHLD}='IGNORE';
  my $stiff;
  while(($stiff = waitpid(-1,&WNOHANG))>0){
    # do something with $stiff if you want
  }
  $SIG{CHLD}=\&REAPER;
}
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For a detailed discussion see sections 6.17 thru 6.19 (espscially 6.19) of the Perl Cookbook.