I offer you a solution and my comments why use hash of 24 elements?
using a hash may brings some problem because if file is the joined day+day you finally get a file corresponding for example a month (I use these kind of reports of automatic station) you can face this situation 2007-01-23 00:01:00,43.3,34.9,30.14,North,359,7,8,72,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 00:06:00,43.2,34.8,30.14,North,354,11,11,72,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 00:12:00,43.2,34.8,30.14,North,1,6,9,72,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 01:11:00,42.9,34.8,30.13,North,1,5,6,73,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 01:17:00,42.9,34.8,30.12,NNW,346,3,7,73,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 01:23:00,43.1,35.0,30.12,North,359,5,11,73,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 02:14:00,43.2,34.1,30.10,North,0,16,16,70,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 02:19:00,43.2,34.1,30.09,North,0,6,13,70,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 02:24:00,43.2,33.7,30.09,North,349,8,14,69,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 23:04:00,43.2,34.1,30.11,North,0,7,12,70,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 23:09:00,43.2,34.1,30.11,North,0,10,17,70,0.00,,,0.00,Wunderground v.1.13, 2007-01-24 00:01:00,43.3,34.9,30.14,North,359,7,8,72,0.00,,,0.00,Wunderground v.1.13, 2007-01-24 00:06:00,43.2,34.8,30.14,North,354,11,11,72,0.00,,,0.00,Wunderground v.1.13, 2007-01-24 00:12:00,43.2,34.8,30.14,North,1,6,9,72,0.00,,,0.00,Wunderground v.1.13, in the 7th line it is a extra white space, because this word is not perfect (noise, or lack of data) then you find a 00: hour after 23 but we using hash met a problem, thereby I offer you perl -lne "($h=$1,print) if (/ (..):/ && $h ne $1)" datos.txt the result is 2007-01-23 00:01:00,43.3,34.9,30.14,North,359,7,8,72,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 01:11:00,42.9,34.8,30.13,North,1,5,6,73,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 02:14:00,43.2,34.1,30.10,North,0,16,16,70,0.00,,,0.00,Wunderground v.1.13, 2007-01-23 23:04:00,43.2,34.1,30.11,North,0,7,12,70,0.00,,,0.00,Wunderground v.1.13, 2007-01-24 00:01:00,43.3,34.9,30.14,North,359,7,8,72,0.00,,,0.00,Wunderground v.1.13, maybe it helps regards
