qr// only compiles regular expressions, not substitution operators. What you've done is compiled a regular expression for matching `s/foo/bar/g'. You can use compiled regexen on the left hand side of an s/// expression, but you can't use one to capture the whole expression.

Now if what you're trying to do is capture an operation or three to be performed on a string that you can select programattically, you possibly could use a hash of coderef like this:

my %mungers = (
            foo => sub { $_[0] =~ s/foo/bar/g },
                fru => sub { $_[0] =~ s/(fru.*)/uc($1)/e },
              );

if( $phase_of_moon > WAXING_GIBBOUS ) {
  $mungers->{foo}->( $string );
} else {
  $mungers->{fru}->( $string );
}
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Update: Feh, as was pointed out that first -> inside the if/else blocks is extraneous. That's what I get for posting while trying to eat breakfast.