That's a great point++.

To that end, if the file is sorted, setting $/ = \'the date'; and reading the first 'record' would move the comparison code from Perl into C, and locate the first record much more quickly. Potentially more quickly that coding a binary chop in Perl?

After reading that first record, you just back up the seek position to the start of the line, reset $/ to "\n", and read on using normal line-by-line semantics until the end of the lines for that day and stop.