Here's something that might work:

#factorial through recursion
use strict;
use warnings;
print "enter number\n";
my $num=<>;
my $fac = &fact($num,1);
print "factorial: $fac\n";
##################
sub fact
{
   my($num1,$flag)=@_;
   if($num1==0) {
     return $flag;
#     print "$flag" and die "brrrrrrrreeeeeee :D\n";
    }
   else
     {fact($num1-1,$num1*$flag);}
}
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It's good (better/best) to use return statement in recursive code!
Also, instead of shift, use @_