my($num1,$flag)=shift;
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my($num1,$flag)=@_;
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In addition to the previously mentioned missing curlies,

• my($num1,$flag)=shift; takes one value from @_ and assigns to the first of two. $flag never gets a value. You want

  my $num1 = shift;
  my $flag = shift;
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  or

  my ($num1, $flag) = @_;
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• Then there's the issue that your program always dies. Return the value you find instead of dying. This will also allow you to remove the print from fact, which is appropriate since finding the factorial of a number has nothing to do with I/O.

• Finally, it's very weird that your factorial function takes two inputs. That's inconsistent with the mathematical function.

Solution:

use strict;
use warnings;

sub fact {
   my ($n) = @_;
   return 1 if $n == 0;
   return $n * fact($n-1);
}

die "usage\n" if !@ARGV;

print(fact($ARGV[0]), "\n");
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Finally, it's very weird that your factorial function takes two inputs. That's inconsistent with the mathematical function.

Yes, but I note that his function looks similar to what a helper function would look like in a language with tail recursion optimization. It should still return a value instead of printing it though.

I have no problem with a helper function having more arguments.

sub fact {
   my ($n) = @_;

   local *_fact = sub {
       my ($n, $prod) = @_;
       return $prod if $n == 0;
       return _fact($n-1, $n*$prod);
   };

   return _fact($n, 1);
}

fact($n);
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In this case, you could even fake it as follows:

sub fact {
   my ($n, $prod) = @_;
   $prod ||= 1;
   return $prod if $n == 0;
   return _fact($n-1, $n*$prod);
}

fact($n);
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But since Perl doesn't do tail recursion optimisation, and since sub calls are relatively slow, you're better off with:

sub fact {
   my ($n) = @_;
   my $prod = 1;
   $prod *= $n-- while $n > 0;
   return $prod;
}

fact($n);
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    (F) Probably means you had a syntax error.  Common reasons include
+:

        A keyword is misspelled.
        A semicolon is missing.
        A comma is missing.
        An opening or closing parenthesis is missing.
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        A closing quote is missing.

    Often there will be another error message associated with the synt
+ax
    error giving more information.  (Sometimes it helps to turn on -w.
+)
    The error message itself often tells you where it was in the line 
+when
    it decided to give up.  Sometimes the actual error is several toke
+ns
    before this, because Perl is good at understanding random input.
    Occasionally the line number may be misleading, and once in a blue
+ moon
    the only way to figure out what's triggering the error is to call
    perl -c repeatedly, chopping away half the program each time to se
+e
    if the error went away.  Sort of the cybernetic version of S<20
    questions>.
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#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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