$a = 'aa';
$a =~ s/a/b/;
print $a

[download]
but cannot use 
$a = 'aa';
$b = 's/a/b/';#or $b = qr(s/a/b/)
$a =~ $b;
print $a

[download]
did not work! have to write like this 
($a) = map {eval $b} $a;

[download]
I used to think qr take care of all regex expressions, but it's sad to find substitution donot work along.

In reply to surprised to find cannot assign s/../../ to a variable by vincentaxhe

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