Following discussions in various branches of this thread, here's a more complete example.

It reports on the following:

• Whatever is output to STDOUT. With the test external command (cat), you'll just see password.
• Whatever is output to STDERR. With 'cat 1>&2', you'll get password here instead; with 'cat -X', an invalid option message, with 'xcat', an invalid command message, and so on.
• An exit value from &run_external_command: either 1 (success), 0 (error, e.g. an invalid command), or <undef> (fatal, e.g. SIGPIPE).
• The command that was run.
• The PID returned by open or an error message.
• The return value of print on success, otherwise an error message.
• The return value of close on success, otherwise an error message.
• Messages from the $SIG{PIPE} handler: either "SIGPIPE not received." or various depending on actions if a SIGPIPE signal was trapped.
• Various other messages under exceptional conditions, e.g. "Died at ...". All of the above is still reported even under such conditions; although values may be empty, or just "<undef>", depending on where the exception occurred.

As well as testing with different commands, I also (temporarily) added lines like the following in different places:

kill PIPE => $$;
kill PIPE => $pid;
kill TERM => $pid;
die 'Test Death';
[download]

Here's the code:

#!/usr/bin/env perl -l

use strict;
use warnings;

use Capture::Tiny qw{capture};
use Try::Tiny;

my $retvals_for;

{
    my ($stdout, $stderr, $exit);

    try {
        ($stdout, $stderr, $exit) = capture \&run_external_command;
    }
    catch {
        die "FATAL! $_" if defined $_;
    }
    finally {
        print_results($stdout, $stderr, $exit);
    };

    die "ERROR! External command failed.\n" unless $exit;
}

sub run_external_command {
    my $external_command = 'cat';
    push @{$retvals_for->{command}}, $external_command;

    {
        my $cmd_pipe;

        push @{$retvals_for->{signal}}, 'SIGPIPE not received.';
        local $SIG{PIPE} = sub {
            pop @{$retvals_for->{signal}};
            push @{$retvals_for->{signal}}, 'SIGPIPE received.', @_;

            if (exists $retvals_for->{open} and $retvals_for->{open}[0
+]) {
                kill TERM => $retvals_for->{open}[0]
                    if kill 0 => $retvals_for->{open}[0];

                my $close_ok = close $cmd_pipe;
                if ($close_ok) {
                    push @{$retvals_for->{signal}}, "Closed pipe: $clo
+se_ok";
                }
                else {
                    push @{$retvals_for->{signal}}, "Can't close pipe:
+ $!, $?";
                }
            }

            die 'SIGPIPE received.';
        };

        my $pid = open $cmd_pipe, '|-', $external_command;
        if (defined $pid) {
            push @{$retvals_for->{open}}, $pid;
        }
        else {
            push @{$retvals_for->{open}}, "Can't open pipe: $!";
            return 0;
        }

        my $print_ok = print $cmd_pipe "password\n";
        if ($print_ok) {
            push @{$retvals_for->{print}}, $print_ok;
        }
        else {
            push @{$retvals_for->{print}}, "Can't print pipe: $!";
            return 0;
        }

        my $close_ok = close $cmd_pipe;
        if ($close_ok) {
            push @{$retvals_for->{close}}, $close_ok;
        }
        else {
            push @{$retvals_for->{close}}, "Can't close pipe: $!, $?";
            return 0;
        }
    }

    return 1;
}

sub print_results {
    my ($stdout, $stderr, $exit) = @_;

    print 'STDOUT:';
    print defined $stdout ? $stdout : '<undef>';
    print 'STDERR:';
    print defined $stderr ? $stderr : '<undef>';
    print 'EXIT:';
    print defined $exit ? $exit : '<undef>';
    print 'Return Values';

    for my $source (qw{command open print close signal}) {
        print ucfirst($source), ':';
        print for @{$retvals_for->{$source}};
    }
}
[download]

Sample output:

STDOUT:
password


STDERR:

EXIT:
1
Return Values
Command:
cat
Open:
5906
Print:
1
Close:
1
Signal:
SIGPIPE not received.
[download]

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