\Q and \E don't work inside interpolations. They only work in regexp literals (m/here/, s/here// and qr/here/). Solutions:

# Text.
$text = 'Renata';
$window =~ /\b\Q$text\E\b/ism
[download]

# Uncompiled regexp.
$text = 'Renata';
$regexp = '\\b' . quotemeta($text) . '\\b';
$window =~ /$regexp/ism
[download]

# Compiled regexp.
$regexp = qr/\b\QRenata\E\b/ism;
$window =~ $regexp
[download]

Note:
The s modifier is useless if you don't use ".".
The m modifier is useless if you don't use "^" or "$".