join on the fly while reading the files in line by line?

This is very easy to do using redo:

$ echo 'somedata data blah \
continuation of previous line \
another continuation of previous line \
stand alone line
stand alone line
multiple line \
continuation of multiple line' | perl -e'
while ( <> ) {
    if ( s/\\\n// ) {
        $_ .= <>;
        redo;
        }
    print;
    }
'
somedata data blah continuation of previous line another continuation 
+of previous line stand alone line
stand alone line
multiple line continuation of multiple line
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