$a='abab';
$b='a(.)';
$c='a$1';

print "\$a $a\n";
print "\$b $b\n";
print "\$c $c\n";

$a =~ s/$b/$c/g ;    # so this is  s/a(.)/a$1/g

print "\$a $a\n";

##</code><code>##

$a abab
$b a(.)
$c a$1
$a a$1a$1

##</code><code>##

$a abab