use warnings;
use strict;
use Data::Dump;

my $re = qr{ \A \s*
    RepeatingGroup \s* = \s*
    (?<GroupID> \b[^\ \t]+\b ) \s*
    ( , \s*
        (?: (?<Flatten> \bFlatten\b )
          | (?<Direction> \b(?:in|out)\b ) )
    \s* ){1,2}
    \z }msx;

dd $_, /$re/ ? \%+ : "FAIL" while <DATA>;

__DATA__
RepeatingGroup = Waiver, Flatten, out
RepeatingGroup = Waiver, out
RepeatingGroup = Waiver, out, Flatten
RepeatingGroup = Waiver , in
RepeatingGroup = Waiver , Flatten
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