You have to escape the backslash:

m{
    \|\\    # the delimiter
    ([^@]*) # everything but a @
    @
    .*?     # garbage inbetween
    \(ACK\)
    (.*)    # the rest
}x;
print "<$1>, <$2>\n";
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See regex tutorial and documentation for more details. (And please enclose your code and data examples with <code>...</code> tags.)