It's a bit messy, but returns what you asked for.

while (<DATA>) {
    $_ =~ s!(b)</it>(<.+?>)<it>(cd)</it>(<.+?>)!$1$2$3$4</it>!;
    print $_;
}

__DATA__
<it>a</it>+2<it>b</it><inf><it>cd</it></inf>
<it>a</it>+2<it>b</it><sup><it>cd</it></sup>
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