$doc->{links} is assumed to be either an array reference or undefined. If it is undefined, the  // defined-or operator (new with Perl version 5.10, see Logical Defined-Or in perlop) will evaluate the  [] anonymous array constructor to create a reference to an empty array. The  @{ } expression will de-reference to a (possibly empty) list either the valid  $doc->{links} reference or the empty reference. (Any expression may be evaluated within a  @{ } dereference.) If the  // operator had not been available, the  || operator would probably have served as well since all genuine references are true and the undefined value is false.

  DB<116> use warnings; $a=undef; print "< @{$a} >"
Use of uninitialized value in array dereference at (eval 49)[multi_per
+l5db.pl:644] line 2.
<  >
  DB<117> use warnings; $a=[]; print "< @{$a} >"
<  >
  DB<118> use warnings; $a=undef//[]; print "< @{$a} >"
<  >
  DB<120> use warnings; $a=[1..3]//[]; print "< @{$a} >"
< 1 2 3 >
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As you can see generating a list from an nonexistent or undef value causes a warning!

Defaulting to an empty array solves this issue.

Thanks for your help :)