Your code is fine. Here it is slightly modified:

use strict;
my %byhour;
my @hours = (
'2005-02-01 14:35:24','2005-02-01 10:35:24',
'2005-02-01 14:35:24','2005-02-01 23:35:24',
'2005-02-01 12:35:24');

foreach (@hours)   # Use default variable $_
{
  /\s(\d\d)/;      # Regex operates on $_ by default
  $byhour{$1}++;   # Will create hash element if needed
}
foreach (0..23) { printf "%2d had %d\n", $_, $byhour{$_}}
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