Something like my $reg = qr{(?$modifiers:$regular)} would probably work...

my $string = "aaaaaaaaa";
$modifiers = "g";
$string =~ s/(?$modifiers:a)/b/; # ??
print $string;
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Although, the more I think about it... I don't think /g in there. /i does.

For that second question, though, I think you'll have to eval like the post above.