In addition to what the others have pointed out, I think you'll need to add an 's' to the end so that you can match across multiple lines, like this:

if ($string =~ /DEFINITION\s\s([\w\s]+)ACCESSION/s){
    print "$1\n";
}
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Also, if you simply want to pick up everything between "DEFINITION " and "ACCESSION", another way to do it without worrying about knowing every possible character is like this:

$string =~ /DEFINITION\s\s(.*?)ACCESSION/s
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