... keep the regex as a string ... has some disavantages ...

Not least of which is that a Regexp object interpolates into another regex as an independent, quantifiable non-capturing group, and an interpolated string does not:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $rs =   'abc|xyz';
 my $rx = qr(abc|xyz);
 ;;
 my $regex = qr( $rs+ $rx+ )x;
 print $regex;
"
(?x-ism: abc|xyz+ (?-xism:abc|xyz)+ )
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The two different representations can have very different effects when interpolated into a larger regex.