$main::SIG{__DIE__} = \&some_function; # when does this get run?
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It gets run at the same time as any other code in the same place would. Give it a try:

#!/usr/bin/perl

use strict;
use warnings;
use feature qw/say/;

sub function {
    say "Howdy!";
}

#die "horribly";

$main::SIG{__DIE__} = \&function;

#die "horribly";

#BEGIN { die "horribly"; }
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Try uncommenting each of the various dies; only the one that dies after the signal handler has been assigned will produce a "Howdy!". No implicit blocks of any kind are involved.

Alternatively, check with -MO=Deparse.

Right, I should have thought to try a simpler example, so thanks for the prompt. :)

In essesnce, the code in question run during "BEGIN" time (sort of in an implict BEGIN block) because that's when the "use" is run. I suppose that makes sense, but I've never really thought about that and I've certainly never read about it ... hence my surprose. I can show that with:


#!/usr/bin/perl
# try.pl

use strict;
use warnings;

use OurCommon;

BEGIN { print "BEGIN in try.pl\n"; }
CHECK { print "CHECK in try.pl\n"; }
INIT { print "INIT in try.pl\n"; }
END { print "END in try.pl\n"; }

print "try.pl main code running\n";
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And with:

#!/usr/bin/perl
# OurCommon.pm

use strict;
use warnings;

package OurCommon;

BEGIN { print "BEGIN in OurCommon.pm\n"; }
CHECK { print "CHECK in OurCommon.pm\n"; }
INIT { print "INIT in OurCommon.pm\n"; }
END { print "END in OurCommon.pm\n"; }

print "OurCommon.pm non-sub code running\n";

sub new { print "new in OurCommon\n" }
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Running that with a simple "perl try.pl" I get:

BEGIN in OurCommon.pm
OurCommon.pm non-sub code running
BEGIN in try.pl
CHECK in try.pl
CHECK in OurCommon.pm
INIT in OurCommon.pm
INIT in try.pl
try.pl main code running
END in try.pl
END in OurCommon.pm

So as we can see, the "non-sub" code in the module file is run while the "use" is processed, which is during the BEGIN phase. That also makes it clear why putting the code in an INIT block fixed it. I'm more educated now, but I really wished the docs made that more clear. :) Looking in "perldoc perlmod", I can see this info is implied, sort of if I squint real hard, but it's really not obvious to me at all while reading that.

Thanks to everyone who replied here!

Looking in "perldoc perlmod", I can see this info is implied, sort of if I squint real hard, but it's really not obvious to me at all while reading that

Well, perlmod says  use Module; is exactly equivalent to   BEGIN { require 'Module.pm'; 'Module'->import; }

so :)