Here's a simpler quicker way; instead of the foreach loop and pattern matches which succeed on each line,

# my @lines = grep { -1 != index $_, $address } @contents;
my @lines = grep { -1 != index $_, "=$address " } @contents;
my @indexes = grep {
    -1 != index $content[$_], "=$address ";
} 0 .. $#contents;
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That gives all the lines containing the target $address.

Update: Skeeve++ is correct, substring amended. Added index search.