Try this:
use strict;
use warnings;
open (OUT, ">newfile.txt");
while(<DATA>){
chomp;
my ($time,$rest)=split(/ --/,$_,2);
my ($hours,$mins,$secs,$usec)=split(/\-/,$time);
$hours=~s/^LT//;
my $newsec=$hours*3600+$mins*60+$secs;
print OUT "LT$newsec --$rest\n";
}
close OUT;
__DATA__
LT19-10-13-400922 --TID1-3039 1--RequestREGISTER
LT19-10-13-405432 --TID1-3039 1--Response100
LT19-10-13-410015 --TID1-3039 1--Response401
LT19-10-13-415481 --TID1-3039 2--RequestREGISTER
LT19-10-13-419800 --TID1-3039 2--Response100
The DATA section can be read from another file.
Output (newfile.txt):
LT69013 --TID1-3039 1--RequestREGISTER
LT69013 --TID1-3039 1--Response100
LT69013 --TID1-3039 1--Response401
LT69013 --TID1-3039 2--RequestREGISTER
LT69013 --TID1-3039 2--Response100
I noticed your time values are all the same. I didn't know if you wanted just the seconds, or the LT+seconds or the rest of each line, so I put them all back in.