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my @list = qw/ 7 5 2 2 /;
my $prod = shift @list;
$prod *= $_ foreach @list;

# bad.  Alters @_
sub head { shift }

# good.  Preserves @_
sub head { $_[0] }

i = j + 1
j = 6

prodList lst =
    if (length lst)==1 then head lst
else head lst*(prodList (tail lst))

sub head { $_[0] }
sub tail { @_[1..$#_] }
...
    ? head @_ 
    : (head @_) * prod_list( tail @_ ) 
}

sub factorial { prod_list( 1 .. $_[0] ) }

#!/usr/bin/perl -w
use strict;
...
sub factorial { prod_list 1 .. $_[0] }

print factorial shift;

sub factorial { 
  my $result = 1;
  $result   *= $_ for 1 .. $_[0];
  $result
}

@new_list =
  map  { $_->[0] }
  sort { $a->[1] <=> $b->[1] }
  map  { [ $_, expensive_func( $_ ) ] }
    @old_list;

# who says Perl has too much punctuation? :)
sub prod_list { 1 == @_ ? $_[0] : $_[0] * prod_list( @_[1..$#_] ) }