Thank you. It worked

#!/volume/perl/bin/perl
use warnings;
use strict;
use DateTime::Duration;

my $first = "00:00:01:04";
my $last  = "00:00:08:861";

sub my_parse_duration {
    my $in = shift;
    my %t;
     @t{qw/hours minutes seconds nanoseconds/}
        = $in=~/\A(\d\d):(\d\d):(\d\d):(\d\d\d?)\z/
            or die "failed to parse '$in'";
    $t{nanoseconds}*=1000000; # ms->ns
    return DateTime::Duration->new(%t);
}
sub my_fmt_duration {
    my @t = shift->in_units('hours','minutes','seconds','nanoseconds')
+;

    $t[3]/=1000000; # ns->ms
    return sprintf("%02d:%02d:%02d:%03d",@t);
}
$first = my_parse_duration($first);
$last  = my_parse_duration($last );
$last->subtract_duration($first);
print my_fmt_duration($last ),"\n";
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