If you want to trigger from the alarm, you'll want to use $SIG{ALRM} rather than INT. You don't have to put $var global it just needs to be in the same scope as the signal handler so you could have

alarm(1);

while(1) {
  my $var = "wibble";
  $SIG{ALRM} = sub { die "borked on $var"; };
}

alarm(0);
[download]

That said I assume that you want to a bit more than just trigger an alarm so it might help if you could give it bit more context