Setting the environment variable PERL_SIGNALS to "unsafe" works, but it must be set before the script is started. What you could do is put something like this at the top of your script:

unless ($ENV {PERL_SIGNALS} && $ENV {PERL_SIGNALS} eq "unsafe") {
    $ENV {PERL_SIGNALS} = "unsafe";
    exec $0, @ARGV;
}
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This will look at your environment, and if the environment variable isn't set to the value you want, it sets it, and execs itself. Now your alarm ought to work (assuming your Perl is 5.8.1 or older).