A stringy regex appearing in a match is treated as between double quotes. That makes perl attempt to interpolate both "@" and "$" followed by legal variable names.

Interpolation is done before the regex is compiled. Since an array being interpolated is joined by the value of the $" variable, I like to use a local value of $" to construct big alternations:

my $alt_foo_re = do {
    local $" = '|';
    qr/@foo/;
};

# same as:
# my $alt_foo_re = do {
#     my $alt_foo = join '|', @foo;
#     qr/$alt_foo/;
# }
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