The basic idea is that you need to convert a time string into a number of seconds. For your specific example, the following will work:

sub to_seconds {
  my $time_string = shift;
  my $secs;
  if ($time_string =~ m/(\d+):(\d+):(\d+)\s*(AM|PM)/i) {
    $secs = $1*3600 + $2*60 + $3 + (lc($4) eq 'pm' ? 12*3600 : 0);
  } else {
    warn "unable to parse this time: $time_string\n";
  }
  return $secs; # returns undef if unable to parse string
}
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From here you should be able to figure out how to determine if two readings are in the same bucket.

If you need to take into account the calendar day, have a look at the str2time routine in Date::Parse.