\ is special in string literals (pieces of code that build strings, incl the replacement expression of s///) and in regex patterns. Escaping it with another \ will cause it to be treated literally.

for (
   '8\\\\.',            # 8\\.
   '8__BACKSLASH__.',
) {
    my $s = $_;
    $s =~ s/\\\\|__BACKSLASH__/\\/g;
    print "$s\n";
}
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8\.
8\.
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