use warnings;
use strict;

# PROOF OF CONCEPT ONLY
# - NO race condition protection or file locking
# - method for killing not entirely reliable and can loop endlessly
#   (see e.g. source of Time::Limit for ideas)

my $PIDFILE = "foo.pid"; # FIXME: use absolute path

$SIG{TERM} = sub { print "exit by signal\n"; exit 0; };

if (-e $PIDFILE) {
    my $oldpid = do { open my $ifh, '<', $PIDFILE or die $!; <$ifh> };
    while ( kill(0,$oldpid) ) {  # is proc running & can we signal it?
        kill 'TERM', $oldpid;
        sleep 1;
    }
}

END { unlink $PIDFILE }
open my $ofh, '>', $PIDFILE or die $!;
print $ofh $$;
close $ofh;

sleep 10; # do heavy work

print "normal exit\n";