I have rewritten your code with a simpler algorithm.

use strict;
use Data::Dumper;

my %records; # hash to store book info based on author
my $data;
while (<DATA>)
{
    chomp;
    $data .= $_;
    if ($_ eq '</ref>') {
        # process what's in the buffer when we see the end tag
        my $rec = process_record($data);
        $records{$rec->{author}} = $rec;
        $data = '';
    }
}

print print Dumper(\%records);

sub process_record
{
    my $rec = shift;
    my %col;

    ($col{author})  = $rec =~ m/<author>\s*([^<]*)(?=<)/g;
    ($col{year})    = $rec =~ m/<year>\s*([^<]*)(?=<)/g;
    ($col{source})  = $rec =~ m/<source>\s*([^<]*)(?=<)/g;
    ($col{id})      = $rec =~ m/<id>\s*([^<]*)(?=<)/g;
    ($col{title})   = $rec =~ m/<title>\s*([^<]*)(?=<)/g;
    my @keywords    = $rec =~ m/<key>\s*([^<]*)(?=<)/g;
    $col{keywords}  = \@keywords;

    return \%col;
}


__DATA__
<ref>
<provnc>
<aulist>
<author> Bin Laden
</aulist>
<year>1990
<source> Cambridge University Press, Cambridge UK, 1st edition 
<id>1
<keywords>
<key>terrorism
<key>whatever
</keywords>
</provnc>
<title> Terrorism
</ref>

<ref>
<provnc>
<aulist>
<author> Sydney
</aulist>
<year>1990
<source> Cambridge University Press, Cambridge UK, 1st edition 
<id>1
<keywords>
<key>nothing
<key>whatever
</keywords>
</provnc>
<title> Terrorism
</ref>
[download]

And the output is as expected -

$VAR1 = {
          'Bin Laden' => {
                           'title' => 'Terrorism',
                           'author' => 'Bin Laden',
                           'keywords' => [
                                           'terrorism',
                                           'whatever'
                                         ],
                           'id' => '1',
                           'year' => '1990',
                           'source' => 'Cambridge University Press, Ca
+mbridge UK, 1st edition '
                         },
          'Sydney' => {
                        'title' => 'Terrorism',
                        'author' => 'Sydney',
                        'keywords' => [
                                        'nothing',
                                        'whatever'
                                      ],
                        'id' => '1',
                        'year' => '1990',
                        'source' => 'Cambridge University Press, Cambr
+idge UK, 1st edition '
                      }
        };
[download]

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