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$SIG{ALRM} = sub { exit; };

alarm 60 * 60 * 2;

open(F, ">$wherever/myscript.pid") or warn "myscript.pid: $!\n";
print F "$$\n";
...
END {
   unlink "$wherever/myscript.pid";
}

[ -r $wherever/myscript.pid ] && kill `cat $wherever/myscript.pid`