perl -E 'my $str = "/regex/i";
  my ($pat, $flags) = $str =~ m{^/(.+)/([imsx]*)\z}
    ? ($1, $2) : die "Bad regex";
  my $qr = qr/(?$flags:$pat)/;
  say qq{Matched "$_" with $qr}
    for grep {$_ =~ $qr} qw(foo bar baz reGex)'

Matched "reGex" with (?^u:(?i:regex))