use Time::Local 'timelocal_nocheck';

## given year+julian date,
my $epoch = timelocal_nocheck $sec, $min, $hour, $julian, 0, $year-1900;

## or given y/m/d (could use normal "timelocal" here)
my $epoch = timelocal_nocheck $sec, $min, $hour, $day, $mon-1, $year-1900;

## add milliseconds on at the end:
$epoch .= ".$milli";