Well, first, if $field == '987' then there is nothing to split by ':', right?

If you can be sure about your data, why not do the following:

while (<THATFILE>) {
  if (/\d+ (\d+):(\d+):(\d+)((,\d+)+)/) {
    my ($hour, $min, $sec, $line) = ($1, $2, $3, $4);
    if ($line =~ /,987/) {
      do-something-with-time-values;
    }
  }
}
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