my $myVar='old yeller';
my $b = 'cat $1 dog $2 mouse $myVar';
$_ 'hello sir, I am wally, the greeter';

# Use a substitution to replace the matched parts with $b
s/(hello).+(wally).+/$b/;

##</code><code>##

s/(hello).+(wally).+/sub { $b =~ s/\$\d+/$$1/g; }/;