my $SUBS='s/AA/BB/g';
$STR = "AAXXXAA";
eval "\$STR =~ $SUBS;";
print $STR, "\n";

##</code><code>##

my $lh_sub = "AA";
my $rh_sub = "BB";
$STR = "AAXXXAA";
$STR =~ s/$lh_sub/$rh_sub/g;
print $STR, "\n";