I wouldn't use Perl for something so trivial - that's easily done in a shell one-liner. I'll give a shell (bash) solution, porting it to Perl is left as an exercise for the reader. Assume the names of the two files are in variables $FILE1 and $FILE2.

  sort -n -k1,1 <(nl -ba -i2 -v1 $FILE1) <(nl -ba -i20 -v0 $FILE2) | c
+ut -f2-
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