Your use of \Q...\E is too general - it also escapes the parentheses you wanted to use for capturing and your wildcards. Try it with:

$line =~ m@\Qonmouseout="cs()">\E(.*)\Q</a>\E@;
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Update: After looking through wws analysis and seeing that my code doesn't work - you also need the /s switch to make . match newlines, as your string contrains newlines:

$line =~ m@\Qonmouseout="cs()">\E(.*)\Q</a>\E@s;
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