I've tried your code but it required a small change to run for me under perl v5.20.3. As it stands, the -t causes it grief:

[11541] START [02@21:54:27] Warning: Use of "-t" without parentheses i
+s ambiguous at ./sigs.pl line 61.
[11541] START [02@21:54:27] Use of ?PATTERN? without explicit operator
+ is deprecated at ./sigs.pl line 61.
[11541] START [02@21:54:27] Search pattern not terminated or ternary o
+perator parsed as search pattern at ./sigs.pl line 61. Leaving...
[download]

So changing line 61 to be this:

logger(INFO, sprintf("We %s connected to a TTY", -t () ? 'ARE' : 'ARE 
+NOT'));
[download]

It then runs and appears to handle the TSTP and subsequent CONT perfectly well. Which version of perl are you running and on which OS?

Hi and thank you for your suggestion that I did missed.

By testing the same code on 5.22.0 I encountered the same ambiguity warning as you. The code was initially written for a Perl 5.16.3 that runs on a Linux RHEL 6. By doing some more tests today, it seems that my problem was probably related to a mix of logs from different processes -a forgotten previous instance and a new one- in the same nohup.out i see different pids

258135 START 11@11:54:00 Setting up signal handlers
258135 RUN 11@11:54:00 Program PID 258135 has started
258135 RUN 11@11:54:00 We ARE NOT connected to a TTY
258135 In loop; sleeping...
257458 In loop; sleeping...
258135 RUN 11@11:54:02 SIGTSTP / CTRL-Z received. Suspending...
257458 In loop; sleeping...
257458 In loop; sleeping...

Anyway i have improved my sscce by adding a proper propagation of the CONT signal to the childs. And I of course noticed that in case of a TERM to a child, it remains as a 'defunct' process until the parent terminates. I know about waitpid() that should fix that but am unsure of its correct use. Where to put it?

here is my upgraded example:

#!/usr/bin/perl

# test_signal.pl - Testing signals handlers
# to test without TTY, you can use 'nohup perl test_signal.pl'
# to test the suspend/continue either
#    in foreground: C-z then 'bg' or 'fg'
#    in background: kill -tstp <pid> / kill -cont <pid>

use strict;
use warnings;
use feature 'say';

use constant {
    FATAL => 0,
    ERROR => 1,
    INFO  => 2,
    DEBUG => 3,
    TRACE => 4
};

$|++;

my $stop_flag = 0;
BEGIN {
    my $LOG_LVL = TRACE;
    logger(INFO, "Setting up signal handlers");
    sub suspend_trap { logger(INFO, "SIGTSTP / CTRL-Z received. Suspen
+ding...");
                       # $SIG{TSTP} = 'IGNORE';
                       $SIG{TSTP} = 'DEFAULT';
                       kill 'TSTP', -(getpgrp $$);
                       # $SIG{TSTP} = \&suspend_trap; # Paranoid - for
+ unreliable signals - see Perl cookbook ch16.17.3
    }
    
    # A handler can be
    # code ref (sub)
    # name of sub
    # 'IGNORE'
    # 'DEFAULT'
    $SIG{__WARN__} = sub { my $m = shift; chomp $m; logger(ERROR, $m);
+ };
    $SIG{__DIE__}  = sub { my $m = shift; chomp $m; logger(FATAL, $m .
+ " Leaving..."); exit; };
    my @childs = ();
    $SIG{HUP}      = sub { logger(INFO, "SIGHUP received. Forking.");
                           my $pid = fork();
                           if ($pid){
                               logger(INFO, "Child PID $pid has been f
+orked.");
                               push @childs, $pid;
                           }
    };
    $SIG{INT}      = sub { logger(INFO, "SIGINT / CTRL-C received (Int
+errupt from keyboard). Leaving..."); $stop_flag++; };
    $SIG{QUIT}     = sub { logger(INFO, "SIGQUIT / CTRL-\\ received (Q
+uit from keyboard). Leaving..."); $stop_flag++; };
    $SIG{ABRT}     = sub { logger(INFO, "SIGABRT received (Probable ab
+normal process termination requested by a library). Leaving..."); $st
+op_flag++; };
    $SIG{TERM}     = sub { logger(INFO, "SIGTERM - External terminatio
+n request. Leaving..."); $stop_flag++; };
    $SIG{TSTP}     = \&suspend_trap;
    $SIG{CONT}     = sub { $SIG{TSTP} = \&suspend_trap;
                           logger(INFO, "SIGCONT received - continue a
+fter suspension.");
                           kill 'CONT', @childs;
    };

    # Log some text, depending on the current log level
    sub logger {
        my ($lvl, $msg) = @_;
        say __stamp($msg) if ($LOG_LVL >= $lvl);
    }
    
    # This sub Copyright (c) 1996,97,98,99,2000,01 by Randal L. Schwar
+tz
    sub __stamp {
        my ($message) = @_;
        my $stamp = sprintf "[%d] %s [%02d@%02d:%02d:%02d] ", $$, ${^G
+LOBAL_PHASE}, (localtime)[3,2,1,0];
        $message =~ s/^/$stamp/gm;
        $message;
    }
}

logger(INFO, "Program PID $$ has started");
logger(INFO, sprintf("We %s connected to a TTY", -t () ? 'ARE' : 'ARE 
+NOT'));
while (!$stop_flag){
    say "[$$] In loop; sleeping...";
    sleep 3;
};
logger(INFO, "Reached normal end. Ciao.");

END { logger(INFO, "That's all folks!") };
[download]

The best programs are the ones written when the programmer is supposed to be working on something else. - Melinda Varian

I know about waitpid() that should fix that but am unsure of its correct use. Where to put it?

Put that into the signal handler for $SIG{CHLD} in the parent. This is the signal a process gets if a subprocess goes away. See perlipc.

perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'