Вы можете использовать Time::Local
и просто добавить .003
к нему:
#!/usr/bin/perl
use strict;
use warnings;
use Time::Local;
my $timestring = "11/05/2010 16:27:26.003";
my ($mon, $d, $y, $h, $min, $s, $fraction) =
$timestring =~ m{(..)/(..)/(....) (..):(..):(..)([.]...)};
$y -= 1900;
$mon--;
my $seconds = timelocal($s, $min, $h, $d, $mon, $y) + $fraction;
print "seconds: $seconds\n";
print "milliseconds: ", $seconds * 1_000, "\n";