[kbenson]

This is something Ruby likely got form Perl,but they cribbed it wrong. Perl's post-conditionals only work on single statements, not blocks. You can use them for functional style statements though. I.e.

    # Not allowed
    for ( 1 .. 10 ) {
        say $_;
    } if 1;
    
    # Allowed
    map { say $_ } ( 1 .. 10 ) if 1;

The idea seems to be that a post-conditional should be simple and obvious by the time you've parsed the statement. A block makes it confusing, as does mixing post control structures

Post-loop structures have the same limitation:

    # Not allowed
    { say "foo"; say $_ } for ( 1 .. 10 );
    
    # Allowed
    say $_ for ( 1 .. 10 );

When used correctly, these can become very succinct and clear.

    my %data = get_data_record_hash("foo");
    
    $data{$_} = update_field(data{$_}) for ('field1','field4',other_field');
    
    # Compare to map, which normally you expect to return values
    map { $data{$_} = update_field(data{$_}) } ('field1','field4',other_field');

The single statement limitation keeps you from going wild in ways that are probably not useful to future readers of the code (including you).