use strict; # Always.
my $logname = "log.txt";
open LOG, $logname or die "Failed to open $logname, $!";
my( $start, $stop );
while( $start = <LOG> )
{
  ($start) = split /\s/, $start;
  $stop  = <LOG> or die "No End Time in LOG!\n";
  ($stop) = split /\s/, $stop;
  my $time_in_seconds = $stop - $start;
  # Do something with $time_in_seconds
  print "$time_in_seconds\n";
}
close LOG or die "Failed to close $logname, $!";