A note of caution. Copying by dereference as shown in examples above is shallow copying only. In a nested data structure (e.g., hash-of-hashes, array-of-arrays, etc.), a reference may be present at any level. (Indeed, in Perl, a nested data structure must, by definition, contain one or more references at one or more levels. See perldsc.) Had the hashes used in the examples above contained a hash/array/whatever reference as a value, the problem would potentially have just been moved down one or more levels. A reference remains a reference to the original referent no matter how many times it is simply copied. 

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my @ra = (1, 2, 3);
 my %hash = ( a => 'ok',  b => \@ra, );
 ;;
 S(\%hash);
 ;;
 dd \@ra;
 dd \%hash;
 ;;
 sub S {
   my $hashref = shift;
   my %hashcopy = %$hashref;
   ;;
   return do_something_with(\%hashcopy);
   }
 ;;
 sub do_something_with {
   my $hashref = shift;
   ;;
   $hashref->{b}[0] = 9;
   $hashref->{b}[1] = 9;
   $hashref->{b}[2] = 9;
   return 1;
   }
"
[9, 9, 9]

{ a => "ok", b => [9, 9, 9] }

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In reply to Re: Global variable unexpectedly modified when passed by reference by AnomalousMonk
in thread Global variable unexpectedly modified when passed by reference by fshrewsb

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