Since you asked for a regex solution: 

local our %linked_counts_by_abbr;

'' =~ m{(?{
    use XML::LibXML qw( );

    my $parser = XML::LibXML->new();
    my $doc = $parser->parse_file($ARGV[0]);
    my $root = $doc->documentElement();

    for my $usage_node (
        $root->findnodes('/BookReferences/BookUsage')
    ) {
        my $abbr         = $usage_node->getAttribute('BookAbbr');
        my $linked_count = $usage_node->getAttribute('LinkedCount');
        $linked_counts_by_abbr{$abbr} += $linked_count;
    }
})}x;

[download]

Personally, I'd drop the regex. 

use XML::LibXML qw( );

my $parser = XML::LibXML->new();
my $doc = $parser->parse_file($ARGV[0]);
my $root = $doc->documentElement();

my %linked_counts_by_abbr;
for my $usage_node (
    $root->findnodes('/BookReferences/BookUsage')
) {
    my $abbr         = $usage_node->getAttribute('BookAbbr');
    my $linked_count = $usage_node->getAttribute('LinkedCount');
    $linked_counts_by_abbr{$abbr} += $linked_count;
}

[download]

XML::LibXML

In reply to Re: Getting various text into diff. variables using REGEXP by ikegami
in thread Getting various text into diff. variables using REGEXP by kdmurphy001

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