Looks like you need to use eval, since you have an executing context.

my $SUBS='s/AA/BB/g';
$STR = "AAXXXAA";
eval "\$STR =~ $SUBS;";
print $STR, "\n";
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On the other hand, if you don't like eval:

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