Those square brackets are the array constructor operator. Nothing to do with regex patterns.

my @anon = REGEX;
my $anon = \@anon;
/&$anon/
[download]

should be

my @anon = REGEX;
my $anon = \@anon;
/@$anon/
[download]

so

/&{[ REGEX ]}/
[download]

should be

/@{[ REGEX ]}/
[download]