If I understand correctly your question, which I'm not sure of, you may be simply after (untested:

for (0..$#ids) {
    my $id=$ids[$_];
    print exists $hash{$id} ?
      "$id => $hash{$id} => $extra[$_]\n" :
      "$id\t$extra[$_]\n";
}
[download]

Depending on your real needs s/exists/defined/, or even just s/exists//.

Two cmts about your code:

• two strings are either equal or not, so it is pointless to first test with eq and then with ne in an elsif clause,
• avoid C-style for loops where possible, as Perl-style ones are more efficient and generally yield clearer code. (See also yesterday's Re: RFC: "Best Practices" code review section.)

Also, here you need to traverse two arrays in parallel: it's a situation where Perl6's zip would come handy. But you may choose your data structures more efficiently instead: if @ids where an AoA containing the extra information too, then the above code may look (more clearly IMHO) like

for (@ids) {
    my ($id,$extra)=@$_;
    print exists $hash{$id} ?
      "$id => $hash{$id} => $extra\n" :
      "$id\t$extra\n";
}
[download]

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