use Time::Local qw/timegm_nocheck/;

my $date_string = "2005301";    # The 301st day of 2005
my ($year,$julian_day) = $date_string =~ /^(\d\d\d\d)(\d\d\d)$/;
my $gmt = timegm_nocheck 0,0,0,$julian_day,0,$year;
# ...  do something with the gmt value