And to add (because you're a newbie) to chady's reply, you need to store somewhere the value of the "time" function, eg:

# snippet...
open(TIMELOG,">timelog.txt") || die $!;
print TIMELOG time;
close(TIMELOG);
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then when you need to see if clock has been set back:

open(TIMELOG,"<timelog.txt") || die $!;
my $stored_time = <TIMELOG>;
close(TIMELOG);

if (time < $stored_time) {
  ... do something because time now is earlier than stored
}
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.02

cLive ;-)