I like Davido's solution but prefer this for clarity, if not the extra characters (anti-golfing?)-

For (0 .. $#{$msg} ) {

    If (msg->[$_] =~ m/\QContent-Transfer-Encoding: 7bit\E/i) {
        If ( next line is blank ) {
            Splice 
            last;

        }
    }    

}
[download]