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<doc>
<elem>A</elem>
<elem>B</elem>
<elem>C</elem>
</doc>

<doc>
<elem>1</elem>
<elem>5</elem>
<elem>10</elem>
</doc>

<doc>
<elem>A</elem>
<elem>BBBBB</elem>
<elem>CCCCCCCCCC</elem>
</doc>

# Open both input files
open A, "<$file_a" or die "Unable to open $file_a: $!\n";
...
}
close A;
close B;