by_val does not modify its parameter ($_[0]), it modifies a copy of its parameter ($var). Without realizing it, you've used the very refactoring tool you are trying to present to us (my $var = shift;).

Using a temporary variable instead of assigning to (or even reading from) a parameter is standard practice in Perl!

Update: Example: 

sub inc_original {
   $_[0]++;
   return $_[0];
}

sub inc_refactored {
   my ($var) = @_;
   $var++;
   return $var;
}

{  my $i = 3;
   my $j = inc_original($i);
   print("$i + 1 = $j\n");    # 4 + 1 = 4
}
{  my $i = 3;
   my $j = inc_refactored($i);
   print("$i + 1 = $j\n");    # 3 + 1 = 4
}

[download]

In reply to Re^3: Refactoring Perl #7 - Remove Assignments to Parameters by ikegami
in thread Refactoring Perl #7 - Remove Assignments to Parameters by agianni

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