You might consider these examples for some clues:

perl6 -e 'my $c; BEGIN { $c = 1 }; say $c;'
1
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As you can see, the declaration of the $c variable can be before the BEGIN block.

Or you can have:

perl6 -e 'my $c; BEGIN $c = 1; say $c;'
1
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Here we see that the BEGIN phaser (or other phasers) does not need to be a block, it can be a simple statement.

In some cases at least, the BEGIN phaser can be even a simple expression:

perl6 -e 'my $c = BEGIN  3 - 2; say $c;'
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Or you can do even this:

perl6 -e 'my $c; say $c; BEGIN $c = 1'
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Now a more complete example somewhat looking like what you're trying to do. I have a short Perl 6 script (file while_done.pl6) in my current directory and want to print the lines where either of the words say and while is present.

perl6 -ne 'my $c; BEGIN {$c = qw<while say>.Set}; my $line = $_; $line
+.say if $_ (elem) $c for $line.words' while_done.pl6
while True {
    say $line;
say 'Done!';
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And this also works the same way:

perl6 -ne 'my $c = BEGIN qw<while say>.Set; my $line = $_; $line.say i
+f $_ (elem) $c for $line.words' while_done.pl6
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I guess the line would be printed twice if either of the searched words appears twice, but that was just a quick example.

BTW, as already mentioned by Anonymous Monk, note that it is probably better to populate a set within the BEGIN block, rather than an array, because, otherwise, Perl will have to coerce the array into a set for each call to the (elem) operator and this might be inefficient if your input file is large.