my $timespec = qr/\d{2}:\d{2}:\d{2},\d+/;
my ($infile, $offset) = @ARGV;
usage() unless defined $offset and $offset =~ /^[+-]?\d+$/; # integer

...

if (my ($start, $end) = /^($timespec)\s-->\s($timespec)/) {
	$_ = time_to_sec($_) += $offset for $start, $end;
	print "$start --> $end\r\n";
}

...