I'm no master of perlguts either, but it occurs to me that there are two places where you are vulnerable to signal changes: 1. when you set the localized handler, 2. when the localization goes out of scope and the old handler is installed.

Your code traps only one of the two vulnerable points.

What happens if you do:

#!/usr/bin/perl

use warnings FATAL => qw( all );
use strict;

use POSIX qw( :signal_h );

$SIG{ALRM} = sub { print "SIGARLM: main handler\n" };

sub f {
        my $sigset = POSIX::SigSet->new;
        my $blockset = POSIX::SigSet->new( SIGALRM );
        my $old_handler = $SIG{ALRM};

        # Install new handler
        sigprocmask(SIG_BLOCK, $blockset, $sigset );
        $SIG{ALRM} = sub { print "SIGALRM\n" }; # line 15
        sigprocmask(SIG_SETMASK, $sigset );

        # spin a bit.
        my $x;
        $x = $_ for 1..10;  

        # Restore old handler.
        sigprocmask(SIG_BLOCK, $blockset, $sigset );
        $SIG{ALRM} = $old_handler;
        sigprocmask(SIG_SETMASK, $sigset );
}

if (fork) {
        # parent
        f while 1;
} else {
        # child
        sleep 1;
        print "child starting\n";
        1 while kill ALRM => getppid;
}
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