You're using (parenthesis) when you should be using {curly-braces}. The first argument to grep is a { BLOCK } that describes how you want to search the items in the following list.

@existing_files = grep { -e $_ } @untested_files;
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This will return true for any item that can be said to "exist" in the file system. You may want to use -f to test if it's a file (versus, say, -d to see if it's a directory), etc. See _X for additional tests you can perform. Note also that $_ is implied, so you can leave that out: { -e }

# alternate syntax
@exiting = grep(-e, @untested);
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See the documentation for grep, which is what I would have done in the first place.