Maybe try:

$pid = open3(\*CHLDIN, \*CHLDOUT, \*CHLDERR, @$cmd);
close(CHLDIN);
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By the way, your usage of eval { .. }; if ($@) { .. } can lead to problems. Consider the eval { ..; 1 } or do { .. } idiom from this example:

# timeout handling
{
    local $@ = "";
                                                                   
    eval {
        local $SIG{ALRM} = sub { die("alarm\n") }; # "\n" required
        alarm(3);

        # potentially long operation
        chomp($input = <STDIN>);

        alarm(0);
        1;
    } or do {
        die($@) unless $@ eq "alarm\n";
                                                                   
        # timed out
        warn("No answer for three seconds.\n");
        $input = "";
    }
}
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