The pure Perl solution will be faster, more portable, and much sexier (substitute your actual filehandle opened with open for <DATA>):

#!/usr/bin/env perl
 
use 5.010;
use warnings;
 
my @times = map { /^(?:top - (..:..:..))/ } <DATA>;
 
say for @times;
 
__DATA__
top - 00:35:18 up
top - 01:23:45 other junk
completely different format altogether
not - 01:23:45 can't fool me!
top - 02:35:46
top - 03:46:07
[download]

Output:

00:35:18
01:23:45
02:35:46
03:46:07
[download]