Pull the filenames out of @ARGV with shift, like so:

perl -e '$fn=shift @ARGV;open FN, $fn;print while (<FN>);print "\nInpu
+t :";print <STDIN>;' num.txt
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Which prints the contents of my "num.txt" file, then prompts for input:

1481422100622192272001352141350602152800661492052301431991681632291972
+09
Input :test
test
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