That's how you go if you want to hold the substitution part in a variable.

my $string = 'Hello sailor';
my $regex = 'Hello (.*)';
my $substitution = '"Goodbye $1"';

$string =~ s/$regex/$substitution/ee;
print "string is now <$string> \n";
[download]

/usr/bin/perl -w /tmp/regex.pl 
string is now <Goodbye sailor>
[download]