Not clear on what you mean by "in minutes"...round up or down or use floating point? Or did you actually mean seconds? Anyway, this does floating point, in what I hope is an educational (or even amusing) way:

$end_time = shift || "16:54:28";
$start_time = shift || "16:54:23";

if (($end_time =~ /(\d+)/)[0] < ($start_time =~ /(\d+)/)[0]
   || ($end_time =~ /(\d+)/)[0] == ($start_time =~ /(\d+)/)[0]
   && (($end_time =~ /(\d+)/g)[1] < ($start_time =~ /(\d+)/g)[1]
      || ($end_time =~ /(\d+)/g)[1] == ($start_time =~ /(\d+)/g)[1]
      && ($end_time =~ /(\d+)/g)[2] < ($start_time =~ /(\d+)/g)[2])) {
   $end_time =~ s/(\d+)/24+$1/e;
}

use List::Util "sum";
$mindiff = sum(map( 60**(2-$_)*(($end_time =~ /(\d+)/g)[$_]-($start_ti
+me =~ /(\d+)/g)[$_]), 0..2 ))/60;

print $mindiff, "minutes (", $mindiff*60, " seconds)\n";
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But if these are really local times, you are going to have trouble over dst boundaries (and some numbers will be ambiguous).