You're looking at the wrong line. The error occurs in s/$delete_candidate//;. Perhaps you want s/\Q$delete_candidate//;?

Update:

Don't use foreach (<FILE>) (which loads the entire file into memory) when while (<DATA>) (which reads the file one line at a time) suffices.

Also, your code is needlessly complicated. In fact, the regular expession that was giving you problems is completely useless. (It's used to modify $_ which is promptly discarded.) Your code simplifies to the following:

# If you need the stuff in @data_out

my @data_out;
while (<DATA>) {
   next if /^\*{4}200[^6]/;
   push @data_out, $_;
}

print @data_out;
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or

# If you don't need @data_out

while (<DATA>) {
   print unless /^\*{4}200[^6]/;
}
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or

# If you want something easy to read and
# don't mind loading the entire file into memory.

print grep { !/^\*{4}200[^6]/ } <DATA>;
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