Another option over using $" is to inline the join in your regex.

my @a = ( 'a', 'x', 'z' );
my $match = qr/^[@{[ join '', @a ]}]{3}$/;
while(<>)
{
  print if /$match/;
}
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You can embed Perl inside any literal strings in this fashion.

Makeshifts last the longest.