To start ...

• ... means fill in the code as appropriate (and, author was too lazy to provide the whole thing not directly related to the point);
• for { ... } is a reference to for loop code inside while loop;
• then, print statement would go right after for loop.

... so  for { ... } print Dumper( \%rec ) ; expands to ...

...
while ( <DATA> ) {
        ...
        for $field ( split ) {
        ...
        }
        print Dumper( \%rec ) ;

        push @AoH, \%rec;
}
...
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