That's pretty compact as is. Here's one possibility, though (untested): 
while ($line1 = <FILE1> && $line2 = <FILE2>) {
    print RESULT $line1, $line2;
}

[download]
The while loop does the equivalent of your defined check.

Update: merlyn points out that && is too high on the precedence scale to work. Use and instead. Also, this code is likely to fail if you hit a line of '0', as the loop actually does a truth-test. Thanks!

In reply to RE: Merging files by chromatic
in thread Merging files by ANKUR

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