my @to_keep = qw{ a c };
my %keepers;
foreach ( @to_keep ) {
    $keepers{$_}++;
}

[download]
... I thought I could do the same thing using map ...

The way to do that with map would be like this: 

my %keepers = map { $_ => 1 } @to_keep;

[download]

The point is that, in this case, you want each iteration in map to return a key/value pair, not just a single value, and the "fat comma" (=>) does that for you.

In reply to Re^3: Hash assignments using map by graff
in thread Hash assignments using map by njcodewarrior

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